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Usage of MySQL's “IF EXISTS”

Posted by: admin November 1, 2017 Leave a comment

Questions:

Here are two statements that I’d like to work, but which return error messages:

IF EXISTS (SELECT * FROM gdata_calendars WHERE `group` =  ? AND id = ?) SELECT 1 ELSE SELECT 0

and

IF ((SELECT COUNT(*) FROM gdata_calendars WHERE `group` =  ? AND id = ?) > 0)  SELECT 1 ELSE SELECT 0;

The question marks are there because I use parametrized, prepared, statements with PHP’s PDO. However, I have also tried executing this with data manually, and it really does not work.

While I’d like to know why each of them doesn’t work, I would prefer to use the first query if it can be made to work.

Answers:

You cannot use IF control block OUTSIDE of functions. So that affects both of your queries.

Turn the EXISTS clause into a subquery instead within an IF function

SELECT IF( EXISTS(
             SELECT *
             FROM gdata_calendars
             WHERE `group` =  ? AND id = ?), 1, 0)

In fact, booleans are returned as 1 or 0

SELECT EXISTS(
         SELECT *
         FROM gdata_calendars
         WHERE `group` =  ? AND id = ?)

Questions:
Answers:

I found the example RichardTheKiwi quite informative.

Just to offer another approach if you’re looking for something like IF EXISTS (SELECT 1 ..) THEN ...

— what I might write in MSSQL

IF EXISTS (SELECT 1 FROM Table WHERE FieldValue='')
BEGIN
    SELECT TableID FROM Table WHERE FieldValue=''
END
ELSE
BEGIN
    INSERT INTO TABLE(FieldValue) VALUES('')
    SELECT SCOPE_IDENTITY() AS TableID
END

— rewritten for MySQL

IF (SELECT 1 = 1 FROM Table WHERE FieldValue='') THEN
BEGIN
    SELECT TableID FROM Table WHERE FieldValue='';
END;
ELSE
BEGIN
    INSERT INTO Table (FieldValue) VALUES('');
    SELECT LAST_INSERT_ID() AS TableID;
END;
END IF;

Questions:
Answers:
if exists(select * from db1.table1 where sno=1 )
begin
select * from db1.table1 where sno=1 
end
else if (select * from db2.table1 where sno=1 )
begin
select * from db2.table1 where sno=1 
end
else
begin
print 'the record does not exits'
end