Usage of MySQL's "IF EXISTS"

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 = ?)

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;