Sql Server: IF EXISTS ; ELSE

Enhancing performance with set-based updates

Running IF EXISTS statements row by row can be a performance predator 🦖. Let's use a set-based approach for update operations, which is like turning the traffic lights green all at once. 🚀 Use a LEFT JOIN nibbled together with ISNULL to do the job:

UPDATE b
SET b.Column = ISNULL(a.MaxValue, '123') -- '123' steps in as a stunt double when MaxValue vanishes.
FROM TableB b
LEFT JOIN (
    SELECT ID, MAX(SomeValue) MaxValue
    FROM TableA
    GROUP BY ID
) a ON b.ID = a.ID


Offering granularity with CASE

For those who like their updates like fine Swiss Watches ⌚ (meticulously controlled), adding the CASE statement can let you dictate values based on conditions:

UPDATE TableB
SET Value = CASE 
                WHEN a.ID IS NOT NULL THEN a.MaxValue -- "Value, meet MaxValue!"
                ELSE '123' -- When ID is moonwalking backward in TableA, '123' saves the day
            END
FROM TableB b
LEFT OUTER JOIN (
    SELECT ID, MAX(SomeValue) AS MaxValue
    FROM TableA
    GROUP BY ID
) a ON b.ID = a.ID


Achieving more efficiency-cum-flexibility with COALESCE

For photo-finish-like results where efficiency and flexibility go hands in hands, COALESCE steps in. It's like a restaurant delivering the first available non-null dish from their menu 🥗:

UPDATE TableB
SET Value = COALESCE(a.MaxValue, '123') -- Agent '123', reporting for duty in absence of MaxValue!
FROM TableB b
LEFT JOIN (
    SELECT ID, MAX(SomeValue) AS MaxValue
    FROM TableA
    GROUP BY ID
) a ON b.ID = a.ID


Harnessing all the corner cases

Always remember to leave no stone unturned. Make sure your IF EXISTS statement caters to all the potential wanderers in your data land:

• When ID throws a party in TableA, get the maximum value from the party
• When ID plays hide-n-seek and can't be found in TableA, recruit the trusty '123' to TableB