SQL conditional 'WHERE' clause

Question

I'm trying to do a conditional AND within a SQL WHERE clause. A pseudo code example is below.

SELECT
    *
FROM [Table]
WHERE
    [A] = [B]
AND
    IF EXISTS
    (
        SELECT TOP 1 1
        FROM [Table2]
        WHERE
            1 = 1
    )
        BEGIN
            --Do conditional filter
            (Table3.[C] = Table.[A])
        END

So, if the if condition is true, the rest of the filtering should be applied. Any help please?

Answer 1

This should cater for the chance of the conditional filter and without

AND
(
    NOT EXISTS
    (
        SELECT TOP 1 1
        FROM [Table2]
        WHERE
            1 = 1
    )
    OR
    (
        EXISTS
        (
            SELECT TOP 1 1
            FROM [Table2]
            WHERE
                1 = 1
        )
        AND
        (
            --Do conditional filter
            (Table3.[C] = Table.[A])
        )
    )
)

Answer 2

In a WHERE clause there is not only AND. You can also use OR, NOT and parentheses. Thus you can express any combination of conditions. In your example you don't want to select any data when there is a table2 entry but no matching table3 entry.

select *
from table1 t1
where a = b
and not
(
   exists (select * from table2)
   and
   not exists (select * from table3 t3 where t3.c = t1.a)
);

Answer 3

how about using a left join, and checking for null? something like this (also pseudo :))

SELECT
    *
FROM [Table]
LEFT JOIN Table2 ON Table2.id = Table.id
WHERE
    [A] = [B]
AND
    Table2.x IS NULL OR Table3.c = Table.A

Answer 4

This will help you : 

DECLARE @SelectQuery    VARCHAR(100)
DECLARE @WhereCondition VARCHAR(50) = ''

SET @SelectQuery = 'SELECT *
                    FROM [Table]
                    WHERE [A] = [B]'

IF EXISTS(SELECT TOP 1 1
            FROM [Table2]
            WHERE 1 = 1
        )
BEGIN
    --Do conditional filter
    SET @WhereCondition = ' AND (Table3.[C] = Table.[A])'
END

EXEC (@SelectQuery + @WhereCondition)

Answer 5

Sorry for the late reply, but I think this is a best way to achieve what you need:

SELECT
    *
FROM [Table]
WHERE
    [A] = [B]
AND
(
    NOT EXISTS
    (
        SELECT NULL
        FROM [Table2]
        WHERE 1 = 1
    )
    OR
        --Do conditional filter
        (Table3.[C] = Table.[A])
)

Note that I used a NOT EXISTS and an ORconditional. This way, if no rows in Table2 satisfy the WHERE, the NOT EXISTS is true and the second condition doesn't matter. Otherwise, the OR will be true depending on the last condition.

I am assuming Table3 is joined to Table in that query, right? Because otherwise of course you couldn't do that last condition. It would be great if you could post a query closer to the one you are actually using.

PS: I guess the 1 = 1 is not the real condition, right? As I asked you in a comment, are both this condition as well as [A] = [B] totally unrelated to the other tables?