SQL Server: IF EXISTS ; ELSE

existsif-statementsqlsql-server

I have a tableA:

Also TableB

ID Code
1
2

Now I want to populate col = code of table B if there exists ID = 2 in tableA. for multiple values , get max value.
else populate it with '123'. Now here is what I used:

if exists (select MAX(value) from #A where id = 2)
 BEGIN
 update #B
 set code = (select MAX(value) from #A where id = 2)
 from #A
 END

 ELSE 

 update #B
 set code = 123
 from #B

I am sure there is some problem in BEGIN;END or in IF EXIST;ELSE.
Basically I want to by-pass the else part if select statement in IF-part exist and vice- versa. For example if select statement of IF=part is:

(select MAX(value) from #A where id = 4)

It should just populate 123, coz ID = 4 do not exist !

Best Solution

EDIT

I want to add the reason that your IF statement seems to not work. When you do an EXISTS on an aggregate, it's always going to be true. It returns a value even if the ID doesn't exist. Sure, it's NULL, but its returning it. Instead, do this:

if exists(select 1 from table where id = 4)

and you'll get to the ELSE portion of your IF statement.

Now, here's a better, set-based solution:

update b
  set code = isnull(a.value, 123)
from #b b
left join (select id, max(value) from #a group by id) a
  on b.id = a.id
where
  b.id = yourid

This has the benefit of being able to run on the entire table rather than individual ids.

Related Solutions

Sql – How to perform an IF…THEN in an SQL SELECT

The CASE statement is the closest to IF in SQL and is supported on all versions of SQL Server.

SELECT CAST(
             CASE
                  WHEN Obsolete = 'N' or InStock = 'Y'
                     THEN 1
                  ELSE 0
             END AS bit) as Saleable, *
FROM Product

You only need to use the CAST operator if you want the result as a Boolean value. If you are happy with an int, this works:

SELECT CASE
            WHEN Obsolete = 'N' or InStock = 'Y'
               THEN 1
               ELSE 0
       END as Saleable, *
FROM Product

CASE statements can be embedded in other CASE statements and even included in aggregates.

SQL Server Denali (SQL Server 2012) adds the IIF statement which is also available in access (pointed out by Martin Smith):

SELECT IIF(Obsolete = 'N' or InStock = 'Y', 1, 0) as Saleable, * FROM Product

Sql – Add a column with a default value to an existing table in SQL Server

Syntax:

ALTER TABLE {TABLENAME} 
ADD {COLUMNNAME} {TYPE} {NULL|NOT NULL} 
CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}
WITH VALUES

Example:

ALTER TABLE SomeTable
        ADD SomeCol Bit NULL --Or NOT NULL.
 CONSTRAINT D_SomeTable_SomeCol --When Omitted a Default-Constraint Name is autogenerated.
    DEFAULT (0)--Optional Default-Constraint.
WITH VALUES --Add if Column is Nullable and you want the Default Value for Existing Records.

Notes:

Optional Constraint Name:
If you leave out CONSTRAINT D_SomeTable_SomeCol then SQL Server will autogenerate
a Default-Contraint with a funny Name like: DF__SomeTa__SomeC__4FB7FEF6

Optional With-Values Statement:
The WITH VALUES is only needed when your Column is Nullable
and you want the Default Value used for Existing Records.
If your Column is NOT NULL, then it will automatically use the Default Value
for all Existing Records, whether you specify WITH VALUES or not.

How Inserts work with a Default-Constraint:
If you insert a Record into SomeTable and do not Specify SomeCol's value, then it will Default to 0.
If you insert a Record and Specify SomeCol's value as NULL (and your column allows nulls),
then the Default-Constraint will not be used and NULL will be inserted as the Value.

Notes were based on everyone's great feedback below.
Special Thanks to:
@Yatrix, @WalterStabosz, @YahooSerious, and @StackMan for their Comments.