MySql create record with IF statement - mysql

Question

SQL Query Forum 20210309
I’m building a Firebase Functions application that talks to a Google Cloud SQL database running MySQL 5.7. I’m trying to retrieve a value from a row in one table and, if it exists (the row or the value), insert a record in a different table.
Based on some examples I found online, my code looks like this:
DECLARE meeting_link varchar(2048) DEFAULT ""; SELECT meeting_link from campaigns where id=2 INTO meeting_link; IF LENGTH(meeting_link) > 0 THEN INSERT INTO clicks (target_id, ip_address, user_agent) VALUES (38, "ip-address", "user-agent") END IF;
In all of the different versions of this I tried, I get an error:
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DECLARE meeting_link varchar(2048) DEFAULT ""; SELECT meeting_link from campaigns' at line 1
Looking around some more, I found posts that say I can’t use DECLARE in anything but stored procedures, but I can use local variables (#ml for example) but I can’t seem to get that working correctly either.
Can someone please help me with the SQL I need for this? I need to create the record only if the record in the query exists and return the meeting_link value to my calling program.

Answer 1

Refer to this documentation page:
https://dev.mysql.com/doc/refman/8.0/en/sql-compound-statements.html
This section describes the syntax for the BEGIN ... END compound statement and other statements that can be used in the body of stored programs
That includes DECLARE and also IF/THEN/ELSE/END constructions. You can't use those outside of stored routines.
Here's a trick you can use instead of a stored routine:
SELECT meeting_link from campaigns where id=2 INTO #meeting_link;
INSERT INTO clicks (target_id, ip_address, user_agent)
SELECT 38, 'ip-address', 'user-agent' FROM dual WHERE LENGTH(#meeting_link) > 0;
You can use #meeting_link which is a user-defined variable, not a declared local variable.
Then instead of using IF, use FROM dual WHERE ... for your condition. The dual table is normally a pseudo-table that doesn't really exist but querying it returns 1 row. But you can make that zero rows if the condition in the WHERE clause isn't satisfied.
So the INSERT will conditionally create either one or zero rows.
Re your comment:
If you need this to be done in a single SQL statement, then one option would be to make that single SQL statement CALL a procedure that you create in your MySQL database. Then at least you could use DECLARE and IF like you were intending.
Another alternative is to combine the two statements I show above like this:
INSERT INTO clicks (target_id, ip_address, user_agent)
SELECT 38, 'ip-address', 'user-agent'
FROM campaigns WHERE id=2 AND LENGTH(meeting_link) > 0;
This works because even though you query the campaigns table, it's not mandatory to select the columns of that table. You can select constant values instead. The conditions in the WHERE clause will make this return either one row or zero rows.