Does SQLite check constraint supports checking against existing data? I just read this article, https://www.sqlitetutorial.net/sqlite-check-constraint/, my impression is the check can only apply on the data being inserted, not including existing data.
Here is my use case, suppose the database already have several rows of integer data and they all follow this rule, no data is less than half of the smallest data. Now I like to enforce this rule on all the data to be inserted.
For example, say, I have the following data already, 10, 11, 12, 15, 16. It is ok to add 6 since it is larger than 5 which is half of the smallest data which is 10; but it should not allow adding 3 because it is less than 5.
Thanks!
From CREATE TABLE/CHECK constraints:
The expression of a CHECK constraint may not contain a subquery.
Since the only way to get the min value of the column is a query this means that it is not possible to do it with a CHECK constraint.
Use a trigger:
CREATE TRIGGER trg_value BEFORE INSERT ON tablename
BEGIN
SELECT
CASE
WHEN NEW.value < (SELECT MIN(value) FROM tablename) / 2
THEN RAISE (ABORT, 'Invalid value')
END;
END;
See the demo.
Related
Since SQLite doesn't support TRUE and FALSE, I have a boolean keyword that stores 0 and 1. For the boolean column in question, I want there to be a check for the number of 1's the column contains and limit the total number for the table.
For example, the table can have columns: name, isAdult. If there are more than 5 adults in the table, the system would not allow a user to add a 6th entry with isAdult = 1. There is no restriction on how many rows the table can contain, since there is no limit on the amount of entries where isAdult = 0.
You can use a trigger to prevent inserting the sixth entry:
CREATE TRIGGER five_adults
BEFORE INSERT ON MyTable
WHEN NEW.isAdult
AND (SELECT COUNT(*)
FROM MyTable
WHERE isAdult
) >= 5
BEGIN
SELECT RAISE(FAIL, "only five adults allowed");
END;
(You might need a similar trigger for UPDATEs.)
The SQL-99 standard would solve this with an ASSERTION— a type of constraint that can validate data changes with respect to an arbitrary SELECT statement. Unfortunately, I don't know any SQL database currently on the market that implements ASSERTION constraints. It's an optional feature of the SQL standard, and SQL implementors are not required to provide it.
A workaround is to create a foreign key constraint so isAdult can be an integer value referencing a lookup table that contains only values 1 through 5. Then also put a UNIQUE constraint on isAdult. Use NULL for "false" when the row is for a user who is not an adult (NULL is ignored by UNIQUE).
Another workaround is to do this in application code. SELECT from the database before changing it, to make sure your change won't break your app's business rules. Normally in a multi-user RDMS this is impossible due to race conditions, but since you're using SQLite you might be the sole user.
I'm trying to add an auto-calculated field in SQL Server 2012 Express, that stores the % of project completion, by calculating the date difference by using:
ALTER TABLE dbo.projects
ADD PercentageCompleted AS (select COUNT(*) FROM projects WHERE project_finish > project_start) * 100 / COUNT(*)
But I am getting this error:
Msg 1046, Level 15, State 1, Line 2
Subqueries are not allowed in this context. Only scalar expressions are allowed.
What am I doing wrong?
Even if it would be possible (it isn't), it is anyway not something you would want to have as a caculated column:
it will be the same value in each row
the entire table would need to be updated after every insert/update
You should consider doing this in a stored procedure or a user defined function instead.Or even better in the business logic of your application,
I don't think you can do that. You could write a trigger to figure it out or do it as part of an update statement.
Are you storing "percentageCompleted" as a duplicated column value in the same table as your project data?
If this is the case, I would not recommend this, because it would duplicate the data.
If you don't care about duplicate data, try something separating the steps out like this:
ALTER TABLE dbo.projects
ADD PercentageCompleted decimal(2,2) --You could also store it as a varchar or char
declare #percentageVariable decimal(2,2)
select #percentageVariable = (select count(*) from projects where Project_finish > project_start) / (select count(*) from projects) -- need to get ratio by completed/total
update projects
set PercentageCompleted = #percentageVariable
this will give you a decimal value in that table, then you can format it on select if you desire to % + PercentageCompleted * 100
When I'm creating tables in an SQLite database, separate sequences are automatically created for each table.
However I want to use one sequence for all tables in my SQLite database and also need to set min and max values (e.g. min=10000 and max=999999) of that sequence (min and max means start value of sequence and maximum value that sequence can increment).
I know this can be done in an Oracle database, but don't know how to do it in SQLite.
Is there any way to do this?
Unfortunately, you cannot do this: SQLite automatically creates sequences for each table in special sqlite_sequence service table.
And even if you somehow forced it to take single sequence as source for all your tables, it would not work the way you expect. For example, in PostgreSQL or Oracle, if you set sequence to value say 1 but table already has filled rows with values 1,2,..., any attempt to insert new rows using that sequence as a source would fail with unique constraint violation.
In SQLite, however, if you manually reset sequence using something like:
UPDATE sqlite_sequence SET seq = 1 WHERE name = 'mytable'
and you already have rows 1,2,3,..., new attempts to insert will NOT fail, but automatically assign this sequence maximum existing value for appropriate auto-increment column, so it can keep going up.
Note that you can use this hack to assign starting value for the sequence, however you cannot make it stop incrementing at some value (unless you watch it using other means).
First of all, this is a bad idea.
The performance of database queries depends on predictability, and by fiddling with the sequence of indexes you are introducing offsets which will confuse the database engine.
However, to achieve this you could try to determine the lowest sequence number which is higher than or equal to any existing sequence number:
SELECT MAX(seq) FROM sqlite_sequence
This needs to be done after each INSERT query, followed by an update of all sequences for all tables:
UPDATE sqlite_sequence SET seq=determined_max
Is there any query available to update only first n records of SQLite DB ..?
The previous answer assumes that the primary key identifier is starting at 1. This would not be the case if rows have been deleted.
http://www.sqlite.org/lang_update.html states:
"If SQLite is built with the SQLITE_ENABLE_UPDATE_DELETE_LIMIT compile-time option then the syntax of the UPDATE statement is extended with optional ORDER BY and LIMIT clauses..."
In this case, you can use this simple query:
UPDATE table SET columns = 'value' WHERE 1 LIMIT n ORDER BY identifier ASC
The other clause might not be needed based on the default ordering scheme.
If you are using a sequential int identifier, UPDATE table SET columns = 'value' WHERE identifier <= n
I want to get a number of rows in my table using max(id). When it returns NULL - if there are no rows in the table - I want to return 0. And when there are rows I want to return max(id) + 1.
My rows are being numbered from 0 and autoincreased.
Here is my statement:
SELECT CASE WHEN MAX(id) != NULL THEN (MAX(id) + 1) ELSE 0 END FROM words
But it is always returning me 0. What have I done wrong?
You can query the actual number of rows withSELECT Count(*) FROM tblName
see https://www.w3schools.com/sql/sql_count_avg_sum.asp
If you want to use the MAX(id) instead of the count, after reading the comments from Pax then the following SQL will give you what you want
SELECT COALESCE(MAX(id)+1, 0) FROM words
In SQL, NULL = NULL is false, you usually have to use IS NULL:
SELECT CASE WHEN MAX(id) IS NULL THEN 0 ELSE (MAX(id) + 1) END FROM words
But, if you want the number of rows, you should just use count(id) since your solution will give 10 if your rows are (0,1,3,5,9) where it should give 5.
If you can guarantee you will always ids from 0 to N, max(id)+1 may be faster depending on the index implementation (it may be faster to traverse the right side of a balanced tree rather than traversing the whole tree, counting.
But that's very implementation-specific and I would advise against relying on it, not least because it locks your performance to a specific DBMS.
Not sure if I understand your question, but max(id) won't give you the number of lines at all. For example if you have only one line with id = 13 (let's say you deleted the previous lines), you'll have max(id) = 13 but the number of rows is 1. The correct (and fastest) solution is to use count(). BTW if you wonder why there's a star, it's because you can count lines based on a criteria.
I got same problem if i understand your question correctly, I want to know the last inserted id after every insert performance in SQLite operation. i tried the following statement:
select * from table_name order by id desc limit 1
The id is the first column and primary key of the table_name, the mentioned statement show me the record with the largest id.
But the premise is u never deleted any row so the numbers of id equal to the numbers of rows.
Extension of VolkerK's answer, to make code a little more readable, you can use AS to reference the count, example below:
SELECT COUNT(*) AS c from profile
This makes for much easier reading in some frameworks, for example, i'm using Exponent's (React Native) Sqlite integration, and without the AS statement, the code is pretty ugly.