JOINS, EXISTS or IN which is better? Few questions on SQL

Question

I have few questions on SQL..

1. How to analyze the performance of a query? Any software, inbuilt features of MSSQL server 2005/2008?

2. What shou

Answer 1

They each behave differently: it is not a performance choice

The only correct and reliable choice is EXISTS or NOT EXISTS that works all the time.

• JOIN may needs DISTINCT
• WHERE/LEFT JOIN would needs correct placement of the filter
• NOT IN fails on NULL

Example:

DECLARE @Parent TABLE (foo int NULL)
INSERT @Parent (foo) VALUES (1)
INSERT @Parent (foo) VALUES (2)
INSERT @Parent (foo) VALUES (3)
INSERT @Parent (foo) VALUES (4)

DECLARE @Child TABLE (bar int NULL, foo int NULL)
INSERT @Child (bar, foo) VALUES (100, 1)
INSERT @Child (bar, foo) VALUES (200, 2)
INSERT @Child (bar, foo) VALUES (201, 2)
INSERT @Child (bar, foo) VALUES (300, NULL)
INSERT @Child (bar, foo) VALUES (301, NULL)
INSERT @Child (bar, foo) VALUES (400, 4)
INSERT @Child (bar, foo) VALUES (500, NULL)

--"positive" checks
SELECT -- multiple "2" = FAIL without DISTINCT
    P.*
FROM
    @Parent P JOIN @Child C ON P.foo = C.foo

SELECT -- correct
    P.*
FROM
    @Parent P
WHERE
    P.foo IN (SELECT c.foo FROM @Child C)

SELECT -- correct
    P.*
FROM
    @Parent P
WHERE
    EXISTS (SELECT * FROM @Child C WHERE P.foo = C.foo)

--"negative" checks
SELECT -- correct
    P.*
FROM
    @Parent P LEFT JOIN @Child C ON P.foo = C.foo
WHERE
    C.foo IS NULL

SELECT -- no rows = FAIL
    P.*
FROM
    @Parent P
WHERE
    P.foo NOT IN (SELECT c.foo FROM @Child C)

SELECT -- correct
    P.*
FROM
    @Parent P
WHERE
    NOT EXISTS (SELECT * FROM @Child C WHERE P.foo = C.foo)

Note: with EXISTS, the SELECT in the subquery is irrelevant as mentioned in ANSI 92 standard...

NOT EXISTS (SELECT * FROM @Child C WHERE P.foo = C.foo)
NOT EXISTS (SELECT NULL FROM @Child C WHERE P.foo = C.foo)
NOT EXISTS (SELECT 1 FROM @Child C WHERE P.foo = C.foo)
NOT EXISTS (SELECT 1/0 FROM @Child C WHERE P.foo = C.foo)