Bits of .NET

Checking if records exist with COUNT(*) > 0 is slow. EXISTS stops at first match.

Slow Way:

IF (SELECT COUNT(*) FROM Orders WHERE CustomerId = 123) > 0
BEGIN
    PRINT 'Customer has orders'
END
-- Counts ALL orders even though we only need to know if ANY exist

Fast Way:

IF EXISTS (SELECT 1 FROM Orders WHERE CustomerId = 123)
BEGIN
    PRINT 'Customer has orders'
END
-- Stops immediately after finding first order

Performance: Customer with 10,000 orders – COUNT takes 50ms, EXISTS takes 0.1ms.