MySQL: Check if a value already exists before trying to update a column

Question

I want to perform a check if the value that I want to update already exists in a table before updating a column.

For example: if I have table called Tokens , with column called refresh_token , and token . I only want to update a blank refresh_token only if their exists no other refresh_token with the same value.

I have this:

IF (NOT EXISTS(SELECT * FROM Tokens WHERE refresh_token = 'C08Rbs'))
          BEGIN
            UPDATE Tokens SET refresh_token='C08Rbs' WHERE id='2'
          END
        END;

but it is returning an error:

ERROR 1064 (42000): 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 'IF (NOT EXISTS(SELECT * FROM Tokens WHERE refresh_token = 'C08Rbs' at line ERROR 1064 (42000): 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 'IF (NOT EXISTS(SELECT * FROM Tokens WHERE refresh_token = 'C08Rbs' at line 1

How should I do this?

NOTE: I am not using a stored procedure. I want to do this using one query.

And is there a way I can do it using an if-else statement??. Because if to execute a statement if the value exists

Answer 1

You can do this in one statement:

UPDATE Tokens
    SET refresh_token = 'C08Rbs'
    WHERE id = '2' AND
          NOT EXISTS (SELECT 1 FROM Tokens WHERE refresh_token = 'C08Rbs');

EDIT:

Ooops. In MySQL, you need to do this with a join :

UPDATE Tokens t CROSS JOIN
       (SELECT COUNT(*) as cnt
        FROM Tokens
        WHERE refresh_token = 'C08Rbs'
       ) tc
    SET refresh_token = 'C08Rbs'
    WHERE id = '2' AND tc.cnt = 0;

Answer 2

Try this:

IF (SELECT tocken_id FROM Tokens WHERE refresh_token = 'C08Rbs') IS NOT NULL THEN
          BEGIN
            UPDATE Tokens SET refresh_token='C08Rbs' WHERE id='2'
          END
END IF;

Where tocken_id is unique field or any field in Tockens table.