UPDATE statement wrapped in an IF EXISTS block

Question

I'm trying to write a DML script that updates a column but I wanted to make sure the column existed first so I wrapped it in a IF EXISTS block

IF EXISTS(SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='Client' AND COLUMN_NAME='IsClarityEnabled') 
BEGIN
    UPDATE Client SET IsClarityEnabled = 1 WHERE ClientID = 21
END

So the weirdness is that it tries to execute the update even if it fails the condition. So column doesn't exist and the UPDATE statement runs and I get an error. Why?

Even stranger is that this does work:

IF EXISTS(SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='Client' AND COLUMN_NAME='IsClarityEnabled') 
BEGIN
    EXEC('UPDATE Client SET IsClarityEnabled = 1 WHERE ClientID = 21')
END

Is there something special about an UPDATE command that causes it to behave this way?

Answer 1

The problem is that the script will be compiled/parsed, and if the column does not exist, you will have a compile/parse error.

Invalid column name 'IsClarityEnabled'.