MySQL-在存储过程中将SELECT用于IF语句条件

Question

I want to execute, in a stored procedure, a certain set of statements if, in table my_table there is exactly one row with value value in column column_name . I have tried the following, but I get a syntax error:

IF ((SELECT COUNT(*) FROM my_table WHERE column_name = value) = 1) THEN
    BEGIN
    END;
END IF;

For context: In my procedure I create a temporary table at some point, where I store a list of values. Then later on in the procedure, I want to check if a given value is present in that temporary table.

Answer 1

I think you might be better to structure it more like this

BEGIN
DECLARE myCOUNT INTEGER;

SELECT COUNT(*) 
INTO myCount
FROM my_table
WHERE column_name=value; 

IF (myCount = 1) THEN
 -- do stuff
END IF;

END;

Answer 2

I'm not sure what you are trying to do, but I'll guess an "upsert" -- update a record if it exists, otherwise insert a new record.

In any case, if you are trying to ensure that name is unique in my_table , then this is not the right approach at all. Instead, declare a unique index/constraint so the database ensures the data integrity:

create unique index unq_my_table_name on my_table(name);

You can then use insert . . . on duplicate key update insert . . . on duplicate key update insert . . . on duplicate key update to modify the records in the database.