Problem: I want to update values (in multiple rows and columns) of a table in a postgres database using R.
I know that the sql update statement can be something like this but I assume looping over a set of such queries is inefficient:
UPDATE table
SET col1 = value1, col2 = value2, ...
WHERE col1 = "some-value"
Question: Is there a function available to only update particular rows (and potentially only a subset of the columns) of the table (similar to dbWriteTable)? If not, can you think of an efficient way/sql query of updating multiple rows in postgres and how to hand over the R object to the sql query?
EDIT: Assuming I have a foreign key and I don't want to turn on the ON DELETE CASCADE option, how could I efficiently update values in multi rows and columns for the parent table (I only want to update parent table, not child table)?
First, I would read in the data that needs to be updated from SQL to R. Second, I would delete the data to be updated in SQL. Third, I would append the updated data from R to SQL.
Related
I have two databases with the same structure. The first is the main one, while the second get updated periodically (in reality I have multiple "secondary" databases that I want to merge one by one into the main one).
The structure of the main and the secondary databases is identical.
I want to periodically dump all new values from the secondary database in the main one. However, the second time I do it, I want to exclude rows that were already copied the first time (and so on).
The tables in all these database have:
an ID column set as PRIMARY KEY going from 1 to N for each database (I suspect this was a mistake, but at the moment I can't change this)
a DATE column, representing a posix timestamp (float)
some other columns
My code looks like this:
ATTACH DATABASE secondary.db AS temp_db
DROP TABLE IF EXISTS my_table_temp
CREATE TABLE my_table_temp AS SELECT * FROM my_table
INSERT INTO main.my_table_temp SELECT * FROM temp_db.my_table
DELETE FROM my_table
INSERT INTO main.my_table SELECT DISTINCT * FROM main.my_table_temp ORDER BY date
DROP TABLE my_table_temp
the problem is that - I suspect due to the repeated ID column - the DISTINCT clause returns me:
UNIQUE constraint failed: my_table.id
However I don't care at all of the ID field that could also be dropped or reset.
NOTES:
the secondary databases are constantly updated by a code that - at the moment - I can't change
I initialize the "main" database copy-pasting one of the secondary to avoid regenerating the whole structure from scratch. Maybe there is a better way of doing this
Apologies if this is a naive question, but I'm very new with SQLite.
Thanks
Following the advice from #forpas, I solved this with the following code:
Assuming the columns to be id,date,col1 and col2
ATTACH DATABASE secondary.db AS temp_db
DROP TABLE IF EXISTS my_table_temp
CREATE TABLE my_table_temp AS SELECT date,col1,col2 FROM my_table
INSERT INTO main.my_table_temp SELECT date,col1,col2 FROM temp_db.my_table
DROP TABLE my_table /* I need to recreate my_table as I've removed a column*/
CREATE TABLE main.my_table AS SELECT DISTINCT date,col1,col2 FROM main.my_table_temp ORDER BY date
DROP TABLE my_table_temp
also, I automatized the extraction of the column names doing
SELECT name FROM PRAGMA_TABLE_INFO('my_table');
This is then passed to the python code running the script and the column id is removed from the list. Note that the second (and following) time I run this code, the column id won't be present in my_table to start with. However this approach allows the code to be the same in the two cases: either if the column id is there or not.
This procedure is then iterated over each table name to fully merge the two databases.
i am planning to write stored procedure (teradata) with bunch of update statements in it .i want to capture number of rows updated for each update statement into an audit table .so can some one help me with SQL?
You can use the ACTIVITY_COUNT variable within your stored procedure. For example:
UPDATE MyTable
SET col1 = 'val';
INSERT INTO MyAuditTable (ActivityCount)
VALUES (:ACTIVITY_COUNT);
Keep in mind this variable is only accessible within your SP.
TD Manual
I am using SQLite and have the following SQL Statement which updates column active with true for just row 28.
update "customer" set "active"='true' where rowid=28
What I would like to do is batch update the entire customer table, active column to true. I would have thought a loop was the best method, but I don't think sqlite supports loops. Anyone advise me how I can update a column in a table so all the records contain the value the in them?
Thanks
update "customer" set "active"='true'
will do the job (just don't specify a where)
I'm copying quite a lot of data from Excel into Access. The trouble is, I have a lookup field and I must select a value from it for each new row. There are about 1000 rows and I wondered if I can somehow fill in those lookups automatically.
Create a linked table to the Excel. Let's call it ExcelLinked.
Create a query with ExcelLinked and your lookup table, let's call it tblLookupItems.
The query will be:
INSERT INTO TargetTable (SELECT ExcelLinked.*, tblLookupItems.ID FROM ExcelLinked INNER JOIN tblLookupItems ON ExcelLinked.LookupItem = tblLookupItems.LookupItem)
However, if there are values in the Excel file that do not exist in the lookup table, you will have to decide whether you are willing to forgo those rows or use a lookup ID of 0 in which cae you should use a LEFT JOIN in the SQL query.
Ok I have a sqlite db, that has roughly 100 rows. It is kind of a strange thing that I'm trying to do, but I need to insert a new row between each of the existing rows.
I have been trying to use the Insert statement as follows, but haven't had any luck:
insert into t1(column1) values("hello") where id%2 == 0
So I'm basically trying to use the %-operator to tell me if the id is even or odd. For every even id number, I'd like to insert a new row.
What am I missing? What can I do differently? How can I insert a new row into every other row and have the index updated as well?
Thanks
Your question assumes that the rows have some kind of built-in order to them, and that you can insert rows between other rows. That's not true.
It is true that rows have an order on disk, and that the id column is usually assigned in order, but that's an implementation detail. When you perform a query, the database is free to return the rows in any order it chooses, unless you specify what you want with an ORDER BY clause.
Now, I'm assuming what you really want is to insert rows between the existing rows in id order. One way to get what you want would look like this:
UPDATE t1 SET id = id * 2
INSERT INTO t1 (id, column) SELECT id+1, "hello" FROM t1
The UPDATE would double the ids of all the existing rows (so 1,2,3 becomes 2,4,6); then the INSERT would perform a query on t1 and use the result to insert a new set of rows with id values one more than the existing rows (so 2,4,6 becomes 3,5,7).
I haven't tested the above statements, so I don't know if they would work or if they require some extra trickery (like a temporary table) since we are querying and updating the same table in one statement. Also I may have made a syntax error.
Don't consider the rows as pre-ordered in the database. A database will store them as they come in, or according to an index. It's your task to order them on retrieval (i.e. when you query for data) according to your needs.