Related
I have a query like this (simplified and anonymised):
SELECT
Department.id,
Department.name,
Department.manager_id,
Employee.name AS manager_name
FROM
Department
LEFT OUTER JOIN Employee
ON Department.manager_id = Employee.id;
The field Department.manager_id may be NULL. If it is non-NULL then it is guaranteed to be a valid id for precisely one row in the Employee table, so the OUTER JOIN is there just for the rows in the Department table where it is NULL.
Here is the problem: old instances of the database do not have this Department.manager_id column at all. In those cases, I would like the query to act as if the field did exist but was always NULL, so e.g. the manager_name field is returned as NULL. If the query only used the Department table then I could just use SELECT * and check for the column in my application, but the JOIN seems to make this impossible. I would prefer not to modify the database, partly so that I can load the database in read only mode. Can this be done just by clever adjustment of the query?
For completeness, here is an answer that does not require munging both possible schemas into one query (but still doesn't need you to actually do the schema migration):
Check for the schema version, and use that to determine which SELECT query to issue (i.e. with or without the manager_id column and JOIN) as a separate step. Here are a few possibilities to determine the schema version:
The ideal situation is that you already keep track of the schema by assigning version numbers to the schema and recording them in the database. Commonly this is done with either:
The user_version pragma.
A table called "Schema" or similar with one row containing the schema version number.
You can directly determine whether the column is present in the table. Two possibilities:
Use the table_info pragma to determine the list of columns in the table.
Use a simple SELECT * FROM Table LIMIT 1 and look at what columns are returned (this is probably better as it is independent of the database engine).
This seems to work:
SELECT
Dept.id,
Dept.name,
Dept.manager_id,
Employee.name AS manager_name
FROM
(SELECT *, NULL AS manager_id FROM Department) AS Dept
LEFT OUTER JOIN Employee
ON Dept.manager_id = Employee.id;
If the manager_id column is present in Department then it is used for the join, whereas if it is not then Dept.manager_id and Employee.name are both NULL.
If I swap the column order in the subquery:
(SELECT NULL AS manager_id, * FROM Department) AS Dept
then the Dept.manager_id and Employee.name are both NULL even if the Department.manager_id column exists, so it seems that Dept.manager_id refers to the first column in the Dept subquery that has that name. It would be good to find a reference in the SQLite documentation saying that this behaviour is guaranteed (or explicitly saying that it is not), but I can't find anything (e.g. in the SELECT or expression pages).
I haven't tried this with other database systems so I don't know if it will work with anything other than SQLite.
I would need to rename a few columns in some tables in a SQLite database.
I know that a similar question has been asked on stackoverflow previously, but it was for SQL in general, and the case of SQLite was not mentioned.
From the SQLite documentation for ALTER TABLE, I gather that it's not possible to do such a thing "easily" (i.e. a single ALTER TABLE statement).
I was wondering someone knew of a generic SQL way of doing such a thing with SQLite.
Note that as of version 3.25.0 released September 2018 you can now use ALTER TABLE to rename a column.
Example to rename Really Bad : Column Name to BetterColumnName
ALTER TABLE your_table
RENAME COLUMN [Really Bad : Column Name] TO BetterColumnName
Original "create new and drop old table" answer below.
Say you have a table and need to rename "colb" to "col_b":
First create the new table with a temporary name, based on the old table definition but with the updated column name:
CREATE TABLE tmp_table_name (
col_a INT
, col_b INT
);
Then copy the contents across from the original table.
INSERT INTO tmp_table_name(col_a, col_b)
SELECT col_a, colb
FROM orig_table_name;
Drop the old table.
DROP TABLE orig_table_name;
Last you rename the temporary table table to the original:
ALTER TABLE tmp_table_name RENAME TO orig_table_name;
Don't forget to re-create indexes, triggers, etc. The documentation gives a fuller picture of the gotchas and caveats.
Wrapping all this in a BEGIN TRANSACTION; and COMMIT; is also probably a good idea.
This was just fixed with 2018-09-15 (3.25.0)
Enhancements the ALTER TABLE command:
Add support for renaming columns within a table using ALTER TABLE table RENAME COLUMN oldname TO newname.
Fix table rename feature so that it also updates references to the renamed table in triggers and views.
You can find the new syntax documented under ALTER TABLE
The RENAME COLUMN TO syntax changes the column-name of table table-name into new-column-name. The column name is changed both within the table definition itself and also within all indexes, triggers, and views that reference the column. If the column name change would result in a semantic ambiguity in a trigger or view, then the RENAME COLUMN fails with an error and no changes are applied.
Image source: https://www.sqlite.org/images/syntax/alter-table-stmt.gif
Example:
CREATE TABLE tab AS SELECT 1 AS c;
SELECT * FROM tab;
ALTER TABLE tab RENAME COLUMN c to c_new;
SELECT * FROM tab;
db-fiddle.com demo
Android Support
As of writing, Android's API 27 is using SQLite package version 3.19.
Based on the current version that Android is using and that this update is coming in version 3.25.0 of SQLite, I would say you have bit of a wait (approximately API 33) before support for this is added to Android.
And, even then, if you need to support any versions older than the API 33, you will not be able to use this.
Digging around, I found this multiplatform (Linux | Mac | Windows) graphical tool called DB Browser for SQLite that actually allows one to rename columns in a very user friendly way!
Edit | Modify Table | Select Table | Edit Field. Click click! Voila!
However, if someone want to share a programmatic way of doing this, I'd be happy to know!
While it is true that there is no ALTER COLUMN, if you only want to rename the column, drop the NOT NULL constraint, or change the data type, you can use the following set of commands:
Note: These commands have the potential to corrupt your database, so make sure you have a backup
PRAGMA writable_schema = 1;
UPDATE SQLITE_MASTER SET SQL = 'CREATE TABLE BOOKS ( title TEXT NOT NULL, publication_date TEXT)' WHERE NAME = 'BOOKS';
PRAGMA writable_schema = 0;
You will need to either close and reopen your connection or vacuum the database to reload the changes into the schema.
For example:
Y:\> sqlite3 booktest
SQLite version 3.7.4
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> create table BOOKS ( title TEXT NOT NULL, publication_date TEXT NOT NULL);
sqlite> insert into BOOKS VALUES ("NULLTEST",null);
Error: BOOKS.publication_date may not be NULL
sqlite> PRAGMA writable_schema = 1;
sqlite> UPDATE SQLITE_MASTER SET SQL = 'CREATE TABLE BOOKS ( title TEXT NOT NULL, publication_date TEXT)' WHERE NAME = 'BOOKS';
sqlite> PRAGMA writable_schema = 0;
sqlite> .q
Y:\> sqlite3 booktest
SQLite version 3.7.4
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> insert into BOOKS VALUES ("NULLTEST",null);
sqlite> .q
REFERENCES FOLLOW:
pragma writable_schema
When this pragma is on, the SQLITE_MASTER tables in which database can be changed using ordinary UPDATE, INSERT, and DELETE statements. Warning: misuse of this pragma can easily result in a corrupt database file.
alter table
SQLite supports a limited subset of ALTER TABLE. The ALTER TABLE command in SQLite allows the user to rename a table or to add a new column to an existing table. It is not possible to rename a column, remove a column, or add or remove constraints from a table.
Recently I had to do that in SQLite3 with a table named points with the colunms id, lon, lat. Erroneusly, when the table was imported, the values for latitude where stored in the lon column and viceversa, so an obvious fix would be to rename those columns. So the trick was:
create table points_tmp as select id, lon as lat, lat as lon from points;
drop table points;
alter table points_tmp rename to points;
I hope this would be useful for you!
CASE 1 : SQLite 3.25.0+
Only the Version 3.25.0 of SQLite supports renaming columns. If your device is meeting this requirement, things are quite simple. The below query would solve your problem:
ALTER TABLE "MyTable" RENAME COLUMN "OldColumn" TO "NewColumn";
CASE 2 : SQLite Older Versions
You have to follow a different Approach to get the result which might be a little tricky
For example, if you have a table like this:
CREATE TABLE student(Name TEXT, Department TEXT, Location TEXT)
And if you wish to change the name of the column Location
Step 1: Rename the original table:
ALTER TABLE student RENAME TO student_temp;
Step 2: Now create a new table student with correct column name:
CREATE TABLE student(Name TEXT, Department TEXT, Address TEXT)
Step 3: Copy the data from the original table to the new table:
INSERT INTO student(Name, Department, Address) SELECT Name, Department, Location FROM student_temp;
Note: The above command should be all one line.
Step 4: Drop the original table:
DROP TABLE student_temp;
With these four steps you can manually change any SQLite table.
Keep in mind that you will also need to recreate any indexes, viewers or triggers on the new table as well.
Quoting the sqlite documentation:
SQLite supports a limited subset of
ALTER TABLE. The ALTER TABLE command
in SQLite allows the user to rename a
table or to add a new column to an
existing table. It is not possible to rename a colum, remove a column, or add or remove constraints from a table.
What you can do of course is, create a new table with the new layout, SELECT * FROM old_table, and fill the new table with the values you'll receive.
First off, this is one of those things that slaps me in the face with surprise: renaming of a column requires creating an entirely new table and copying the data from the old table to the new table...
The GUI I've landed on to do SQLite operations is Base. It's got a nifty Log window that shows all the commands that have been executed. Doing a rename of a column via Base populates the log window with the necessary commands:
These can then be easily copied and pasted where you might need them. For me, that's into an ActiveAndroid migration file. A nice touch, as well, is that the copied data only includes the SQLite commands, not the timestamps, etc.
Hopefully, that saves some people time.
change table column < id > to < _id >
String LastId = "id";
database.execSQL("ALTER TABLE " + PhraseContract.TABLE_NAME + " RENAME TO " + PhraseContract.TABLE_NAME + "old");
database.execSQL("CREATE TABLE " + PhraseContract.TABLE_NAME
+"("
+ PhraseContract.COLUMN_ID + " INTEGER PRIMARY KEY,"
+ PhraseContract.COLUMN_PHRASE + " text ,"
+ PhraseContract.COLUMN_ORDER + " text ,"
+ PhraseContract.COLUMN_FROM_A_LANG + " text"
+")"
);
database.execSQL("INSERT INTO " +
PhraseContract.TABLE_NAME + "("+ PhraseContract.COLUMN_ID +" , "+ PhraseContract.COLUMN_PHRASE + " , "+ PhraseContract.COLUMN_ORDER +" , "+ PhraseContract.COLUMN_FROM_A_LANG +")" +
" SELECT " + LastId +" , "+ PhraseContract.COLUMN_PHRASE + " , "+ PhraseContract.COLUMN_ORDER +" , "+ PhraseContract.COLUMN_FROM_A_LANG +
" FROM " + PhraseContract.TABLE_NAME + "old");
database.execSQL("DROP TABLE " + PhraseContract.TABLE_NAME + "old");
Create a new column with the desired column name: COLNew.
ALTER TABLE {tableName} ADD COLUMN COLNew {type};
Copy contents of old column COLOld to new column COLNew.
INSERT INTO {tableName} (COLNew) SELECT {COLOld} FROM {tableName}
Note: brackets are necessary in above line.
As mentioned before, there is a tool SQLite Database Browser, which does this. Lyckily, this tool keeps a log of all operations performed by the user or the application. Doing this once and looking at the application log, you will see the code involved. Copy the query and paste as required. Worked for me. Hope this helps
From the official documentation
A simpler and faster procedure can optionally be used for some changes that do no affect the on-disk content in any way. The following simpler procedure is appropriate for removing CHECK or FOREIGN KEY or NOT NULL constraints, renaming columns, or adding or removing or changing default values on a column.
Start a transaction.
Run PRAGMA schema_version to determine the current schema version number. This number will be needed for step 6 below.
Activate schema editing using PRAGMA writable_schema=ON.
Run an UPDATE statement to change the definition of table X in the sqlite_master table: UPDATE sqlite_master SET sql=... WHERE type='table' AND name='X';
Caution: Making a change to the sqlite_master table like this will render the database corrupt and unreadable if the change contains a syntax error. It is suggested that careful testing of the UPDATE statement be done on a separate blank database prior to using it on a database containing important data.
If the change to table X also affects other tables or indexes or triggers are views within schema, then run UPDATE statements to modify those other tables indexes and views too. For example, if the name of a column changes, all FOREIGN KEY constraints, triggers, indexes, and views that refer to that column must be modified.
Caution: Once again, making changes to the sqlite_master table like this will render the database corrupt and unreadable if the change contains an error. Carefully test of this entire procedure on a separate test database prior to using it on a database containing important data and/or make backup copies of important databases prior to running this procedure.
Increment the schema version number using PRAGMA schema_version=X where X is one more than the old schema version number found in step 2 above.
Disable schema editing using PRAGMA writable_schema=OFF.
(Optional) Run PRAGMA integrity_check to verify that the schema changes did not damage the database.
Commit the transaction started on step 1 above.
One option, if you need it done in a pinch, and if your initial column was created with a default, is to create the new column you want, copy the contents over to it, and basically "abandon" the old column (it stays present, but you just don't use/update it, etc.)
ex:
alter table TABLE_NAME ADD COLUMN new_column_name TYPE NOT NULL DEFAULT '';
update TABLE_NAME set new_column_name = old_column_name;
update TABLE_NAME set old_column_name = ''; -- abandon old column, basically
This leaves behind a column (and if it was created with NOT NULL but without a default, then future inserts that ignore it might fail), but if it's just a throwaway table, the tradeoffs might be acceptable. Otherwise use one of the other answers mentioned here, or a different database that allows columns to be renamed.
need to rename a few columns in some tables
Another way is to use multiple SQLite3 commands to "rename" a column,
in "some" tables, repeat as needed:
.output tmp
SELECT "ALTER TABLE """|| sqlite_master.name ||""" RENAME COLUMN old_name TO new_name;" FROM sqlite_master
WHERE type = "table" AND sqlite_master.name NOT LIKE 'sqlite_%';
.read tmp
source
Since version 2018-09-15 (3.25.0)
sqlite supports renaming columns
https://sqlite.org/changes.html
sqlite3 yourdb .dump > /tmp/db.txt
edit /tmp/db.txt change column name in Create line
sqlite2 yourdb2 < /tmp/db.txt
mv/move yourdb2 yourdb
Maybe i should do this in C# but i have more then one row with linkId X. I would like to remove it but i am unsure how. In code i could just use a foreach from 0 to n and remove any found rows with a greater (or !=) id but thats in code. Is there a less difficult way of doing it using sqlite?
Assuming the table's name is tableName and there is a primary key field named id, the following sql would do it. I think the following SQL query is general enough and should be able to be executed under any database engine.
delete from tableName
where id not in (
select min(id) from tableName
group by linkId
)
I would need to rename a few columns in some tables in a SQLite database.
I know that a similar question has been asked on stackoverflow previously, but it was for SQL in general, and the case of SQLite was not mentioned.
From the SQLite documentation for ALTER TABLE, I gather that it's not possible to do such a thing "easily" (i.e. a single ALTER TABLE statement).
I was wondering someone knew of a generic SQL way of doing such a thing with SQLite.
Note that as of version 3.25.0 released September 2018 you can now use ALTER TABLE to rename a column.
Example to rename Really Bad : Column Name to BetterColumnName:
ALTER TABLE your_table
RENAME COLUMN "Really Bad : Column Name" TO BetterColumnName
According to keywords the use of "double-quotes" is the standard way
Original "create new and drop old table" answer below.
Say you have a table and need to rename "colb" to "col_b":
First create the new table with a temporary name, based on the old table definition but with the updated column name:
CREATE TABLE tmp_table_name (
col_a INT
, col_b INT
);
Then copy the contents across from the original table.
INSERT INTO tmp_table_name(col_a, col_b)
SELECT col_a, colb
FROM orig_table_name;
Drop the old table.
DROP TABLE orig_table_name;
Last you rename the temporary table table to the original:
ALTER TABLE tmp_table_name RENAME TO orig_table_name;
Don't forget to re-create indexes, triggers, etc. The documentation gives a fuller picture of the gotchas and caveats.
Wrapping all this in a BEGIN TRANSACTION; and COMMIT; is also probably a good idea.
This was just fixed with 2018-09-15 (3.25.0)
Enhancements the ALTER TABLE command:
Add support for renaming columns within a table using ALTER TABLE table RENAME COLUMN oldname TO newname.
Fix table rename feature so that it also updates references to the renamed table in triggers and views.
You can find the new syntax documented under ALTER TABLE
The RENAME COLUMN TO syntax changes the column-name of table table-name into new-column-name. The column name is changed both within the table definition itself and also within all indexes, triggers, and views that reference the column. If the column name change would result in a semantic ambiguity in a trigger or view, then the RENAME COLUMN fails with an error and no changes are applied.
Image source: https://www.sqlite.org/images/syntax/alter-table-stmt.gif
Example:
CREATE TABLE tab AS SELECT 1 AS c;
SELECT * FROM tab;
ALTER TABLE tab RENAME COLUMN c to c_new;
SELECT * FROM tab;
db-fiddle.com demo
Android Support
As of writing, Android's API 27 is using SQLite package version 3.19.
Based on the current version that Android is using and that this update is coming in version 3.25.0 of SQLite, I would say you have bit of a wait (approximately API 33) before support for this is added to Android.
And, even then, if you need to support any versions older than the API 33, you will not be able to use this.
Digging around, I found this multiplatform (Linux | Mac | Windows) graphical tool called DB Browser for SQLite that actually allows one to rename columns in a very user friendly way!
Edit | Modify Table | Select Table | Edit Field. Click click! Voila!
However, if someone want to share a programmatic way of doing this, I'd be happy to know!
While it is true that there is no ALTER COLUMN, if you only want to rename the column, drop the NOT NULL constraint, or change the data type, you can use the following set of commands:
Note: These commands have the potential to corrupt your database, so make sure you have a backup
PRAGMA writable_schema = 1;
UPDATE SQLITE_MASTER SET SQL = 'CREATE TABLE BOOKS ( title TEXT NOT NULL, publication_date TEXT)' WHERE NAME = 'BOOKS';
PRAGMA writable_schema = 0;
You will need to either close and reopen your connection or vacuum the database to reload the changes into the schema.
For example:
Y:\> sqlite3 booktest
SQLite version 3.7.4
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> create table BOOKS ( title TEXT NOT NULL, publication_date TEXT NOT NULL);
sqlite> insert into BOOKS VALUES ("NULLTEST",null);
Error: BOOKS.publication_date may not be NULL
sqlite> PRAGMA writable_schema = 1;
sqlite> UPDATE SQLITE_MASTER SET SQL = 'CREATE TABLE BOOKS ( title TEXT NOT NULL, publication_date TEXT)' WHERE NAME = 'BOOKS';
sqlite> PRAGMA writable_schema = 0;
sqlite> .q
Y:\> sqlite3 booktest
SQLite version 3.7.4
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> insert into BOOKS VALUES ("NULLTEST",null);
sqlite> .q
REFERENCES FOLLOW:
pragma writable_schema
When this pragma is on, the SQLITE_MASTER tables in which database can be changed using ordinary UPDATE, INSERT, and DELETE statements. Warning: misuse of this pragma can easily result in a corrupt database file.
alter table
SQLite supports a limited subset of ALTER TABLE. The ALTER TABLE command in SQLite allows the user to rename a table or to add a new column to an existing table. It is not possible to rename a column, remove a column, or add or remove constraints from a table.
Recently I had to do that in SQLite3 with a table named points with the colunms id, lon, lat. Erroneusly, when the table was imported, the values for latitude where stored in the lon column and viceversa, so an obvious fix would be to rename those columns. So the trick was:
create table points_tmp as select id, lon as lat, lat as lon from points;
drop table points;
alter table points_tmp rename to points;
I hope this would be useful for you!
CASE 1 : SQLite 3.25.0+
Only the Version 3.25.0 of SQLite supports renaming columns. If your device is meeting this requirement, things are quite simple. The below query would solve your problem:
ALTER TABLE "MyTable" RENAME COLUMN "OldColumn" TO "NewColumn";
CASE 2 : SQLite Older Versions
You have to follow a different Approach to get the result which might be a little tricky
For example, if you have a table like this:
CREATE TABLE student(Name TEXT, Department TEXT, Location TEXT)
And if you wish to change the name of the column Location
Step 1: Rename the original table:
ALTER TABLE student RENAME TO student_temp;
Step 2: Now create a new table student with correct column name:
CREATE TABLE student(Name TEXT, Department TEXT, Address TEXT)
Step 3: Copy the data from the original table to the new table:
INSERT INTO student(Name, Department, Address) SELECT Name, Department, Location FROM student_temp;
Note: The above command should be all one line.
Step 4: Drop the original table:
DROP TABLE student_temp;
With these four steps you can manually change any SQLite table.
Keep in mind that you will also need to recreate any indexes, viewers or triggers on the new table as well.
Quoting the sqlite documentation:
SQLite supports a limited subset of
ALTER TABLE. The ALTER TABLE command
in SQLite allows the user to rename a
table or to add a new column to an
existing table. It is not possible to rename a colum, remove a column, or add or remove constraints from a table.
What you can do of course is, create a new table with the new layout, SELECT * FROM old_table, and fill the new table with the values you'll receive.
First off, this is one of those things that slaps me in the face with surprise: renaming of a column requires creating an entirely new table and copying the data from the old table to the new table...
The GUI I've landed on to do SQLite operations is Base. It's got a nifty Log window that shows all the commands that have been executed. Doing a rename of a column via Base populates the log window with the necessary commands:
These can then be easily copied and pasted where you might need them. For me, that's into an ActiveAndroid migration file. A nice touch, as well, is that the copied data only includes the SQLite commands, not the timestamps, etc.
Hopefully, that saves some people time.
change table column < id > to < _id >
String LastId = "id";
database.execSQL("ALTER TABLE " + PhraseContract.TABLE_NAME + " RENAME TO " + PhraseContract.TABLE_NAME + "old");
database.execSQL("CREATE TABLE " + PhraseContract.TABLE_NAME
+"("
+ PhraseContract.COLUMN_ID + " INTEGER PRIMARY KEY,"
+ PhraseContract.COLUMN_PHRASE + " text ,"
+ PhraseContract.COLUMN_ORDER + " text ,"
+ PhraseContract.COLUMN_FROM_A_LANG + " text"
+")"
);
database.execSQL("INSERT INTO " +
PhraseContract.TABLE_NAME + "("+ PhraseContract.COLUMN_ID +" , "+ PhraseContract.COLUMN_PHRASE + " , "+ PhraseContract.COLUMN_ORDER +" , "+ PhraseContract.COLUMN_FROM_A_LANG +")" +
" SELECT " + LastId +" , "+ PhraseContract.COLUMN_PHRASE + " , "+ PhraseContract.COLUMN_ORDER +" , "+ PhraseContract.COLUMN_FROM_A_LANG +
" FROM " + PhraseContract.TABLE_NAME + "old");
database.execSQL("DROP TABLE " + PhraseContract.TABLE_NAME + "old");
Create a new column with the desired column name: COLNew.
ALTER TABLE {tableName} ADD COLUMN COLNew {type};
Copy contents of old column COLOld to new column COLNew.
INSERT INTO {tableName} (COLNew) SELECT {COLOld} FROM {tableName}
Note: brackets are necessary in above line.
As mentioned before, there is a tool SQLite Database Browser, which does this. Lyckily, this tool keeps a log of all operations performed by the user or the application. Doing this once and looking at the application log, you will see the code involved. Copy the query and paste as required. Worked for me. Hope this helps
From the official documentation
A simpler and faster procedure can optionally be used for some changes that do no affect the on-disk content in any way. The following simpler procedure is appropriate for removing CHECK or FOREIGN KEY or NOT NULL constraints, renaming columns, or adding or removing or changing default values on a column.
Start a transaction.
Run PRAGMA schema_version to determine the current schema version number. This number will be needed for step 6 below.
Activate schema editing using PRAGMA writable_schema=ON.
Run an UPDATE statement to change the definition of table X in the sqlite_master table: UPDATE sqlite_master SET sql=... WHERE type='table' AND name='X';
Caution: Making a change to the sqlite_master table like this will render the database corrupt and unreadable if the change contains a syntax error. It is suggested that careful testing of the UPDATE statement be done on a separate blank database prior to using it on a database containing important data.
If the change to table X also affects other tables or indexes or triggers are views within schema, then run UPDATE statements to modify those other tables indexes and views too. For example, if the name of a column changes, all FOREIGN KEY constraints, triggers, indexes, and views that refer to that column must be modified.
Caution: Once again, making changes to the sqlite_master table like this will render the database corrupt and unreadable if the change contains an error. Carefully test of this entire procedure on a separate test database prior to using it on a database containing important data and/or make backup copies of important databases prior to running this procedure.
Increment the schema version number using PRAGMA schema_version=X where X is one more than the old schema version number found in step 2 above.
Disable schema editing using PRAGMA writable_schema=OFF.
(Optional) Run PRAGMA integrity_check to verify that the schema changes did not damage the database.
Commit the transaction started on step 1 above.
One option, if you need it done in a pinch, and if your initial column was created with a default, is to create the new column you want, copy the contents over to it, and basically "abandon" the old column (it stays present, but you just don't use/update it, etc.)
ex:
alter table TABLE_NAME ADD COLUMN new_column_name TYPE NOT NULL DEFAULT '';
update TABLE_NAME set new_column_name = old_column_name;
update TABLE_NAME set old_column_name = ''; -- abandon old column, basically
This leaves behind a column (and if it was created with NOT NULL but without a default, then future inserts that ignore it might fail), but if it's just a throwaway table, the tradeoffs might be acceptable. Otherwise use one of the other answers mentioned here, or a different database that allows columns to be renamed.
need to rename a few columns in some tables
Another way is to use multiple SQLite3 commands to "rename" a column,
in "some" tables, repeat as needed:
.output tmp
SELECT "ALTER TABLE """|| sqlite_master.name ||""" RENAME COLUMN old_name TO new_name;" FROM sqlite_master
WHERE type = "table" AND sqlite_master.name NOT LIKE 'sqlite_%';
.read tmp
source
Since version 2018-09-15 (3.25.0)
sqlite supports renaming columns
https://sqlite.org/changes.html
sqlite3 yourdb .dump > /tmp/db.txt
edit /tmp/db.txt change column name in Create line
sqlite2 yourdb2 < /tmp/db.txt
mv/move yourdb2 yourdb
I want to find out, with an SQL query, whether an index is UNIQUE or not. I'm using SQLite 3.
I have tried two approaches:
SELECT * FROM sqlite_master WHERE name = 'sqlite_autoindex_user_1'
This returns information about the index ("type", "name", "tbl_name", "rootpage" and "sql"). Note that the sql column is empty when the index is automatically created by SQLite.
PRAGMA index_info(sqlite_autoindex_user_1);
This returns the columns in the index ("seqno", "cid" and "name").
Any other suggestions?
Edit: The above example is for an auto-generated index, but my question is about indexes in general. For example, I can create an index with "CREATE UNIQUE INDEX index1 ON visit (user, date)". It seems no SQL command will show if my new index is UNIQUE or not.
PRAGMA INDEX_LIST('table_name');
Returns a table with 3 columns:
seq Unique numeric ID of index
name Name of the index
unique Uniqueness flag (nonzero if UNIQUE index.)
Edit
Since SQLite 3.16.0 you can also use table-valued pragma functions which have the advantage that you can JOIN them to search for a specific table and column. See #mike-scotty's answer.
Since noone's come up with a good answer, I think the best solution is this:
If the index starts with "sqlite_autoindex", it is an auto-generated index for a single UNIQUE column
Otherwise, look for the UNIQUE keyword in the sql column in the table sqlite_master, with something like this:
SELECT * FROM sqlite_master WHERE type = 'index' AND sql LIKE '%UNIQUE%'
you can programmatically build a select statement to see if any tuples point to more than one row. If you get back three columns, foo, bar and baz, create the following query
select count(*) from t
group by foo, bar, baz
having count(*) > 1
If that returns any rows, your index is not unique, since more than one row maps to the given tuple. If sqlite3 supports derived tables (I've yet to have the need, so I don't know off-hand), you can make this even more succinct:
select count(*) from (
select count(*) from t
group by foo, bar, baz
having count(*) > 1
)
This will return a single row result set, denoting the number of duplicate tuple sets. If positive, your index is not unique.
You are close:
1) If the index starts with "sqlite_autoindex", it is an auto-generated index for the primary key . However, this will be in the sqlite_master or sqlite_temp_master tables depending depending on whether the table being indexed is temporary.
2) You need to watch out for table names and columns that contain the substring unique, so you want to use:
SELECT * FROM sqlite_master WHERE type = 'index' AND sql LIKE 'CREATE UNIQUE INDEX%'
See the sqlite website documentation on Create Index
As of sqlite 3.16.0 you could also use pragma functions:
SELECT distinct il.name
FROM sqlite_master AS m,
pragma_index_list(m.name) AS il,
pragma_index_info(il.name) AS ii
WHERE m.type='table' AND il.[unique] = 1;
The above statement will list all names of unique indexes.
SELECT DISTINCT m.name as table_name, ii.name as column_name
FROM sqlite_master AS m,
pragma_index_list(m.name) AS il,
pragma_index_info(il.name) AS ii
WHERE m.type='table' AND il.[unique] = 1;
The above statement will return all tables and their columns if the column is part of a unique index.
From the docs:
The table-valued functions for PRAGMA feature was added in SQLite version 3.16.0 (2017-01-02). Prior versions of SQLite cannot use this feature.