Usage of MySQL's “IF EXISTS”

Question

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 = ?) SELE         


        

Answer 1

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 = ?)

Answer 2

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;

Answer 3

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

Answer 4

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

For Detail explanation you can visit here

Answer 5

The accepted answer works well and one can also just use the

If Exists (...) Then ... End If; 

syntax in Mysql procedures (if acceptable for circumstance) and it will behave as desired/expected. Here's a link to a more thorough source/description: https://dba.stackexchange.com/questions/99120/if-exists-then-update-else-insert

One problem with the solution by @SnowyR is that it does not really behave like "If Exists" in that the (Select 1 = 1 ...) subquery could return more than one row in some circumstances and so it gives an error. I don't have permissions to respond to that answer directly so I thought I'd mention it here in case it saves someone else the trouble I experienced and so others might know that it is not an equivalent solution to MSSQLServer "if exists"!