Mixed Joins

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Mixed Joins

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November 6, 2018 at 10:21 pm

#372953

Comments posted to this topic are about the item Mixed Joins

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Stewart "Arturius" Campbell SSC Guru Points: 72497 More actions · Answer 1

Nice question, thanks Ben

Trying to explain this to the uninitiated is not simple....

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Tom Litt SSC Journeyman Points: 75 More actions · Answer 2

Definitely a good question! I've seen lots of people query this behaviour. And then trying to explain to them how to "fix" it to get what they intended....

Andrew Watson-478275 SSCarpal Tunnel Points: 4652 More actions · Answer 3

Of course, in a case sensitive database, the answer is "none of the above". table_a <> Table_a <> TABLE_A.

ChrisM@Work SSC Guru Points: 186127 More actions · Answer 4

Force the join between B and C to take place first, and you change the result:
SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
INNER JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID
ON TABLE_A.ID = TABLE_B.ID;
-- 3 rows

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

call.copse SSCoach Points: 17958 More actions · Answer 5

ChrisM@Work - Wednesday, November 7, 2018 3:36 AM
Force the join between B and C to take place first, and you change the result:
SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
INNER JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID
ON TABLE_A.ID = TABLE_B.ID;
-- 3 rows

I get what you're saying, but wouldn't it be clearer to left join to C?

ChrisM@Work SSC Guru Points: 186127 More actions · Answer 6

call.copse - Wednesday, November 7, 2018 3:50 AM
ChrisM@Work - Wednesday, November 7, 2018 3:36 AM
Force the join between B and C to take place first, and you change the result:
SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
INNER JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID
ON TABLE_A.ID = TABLE_B.ID;
-- 3 rows
I get what you're saying, but wouldn't it be clearer to left join to C?

No, because the queries are not logically equivalent:
SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
INNER JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID
ON TABLE_A.ID = TABLE_B.ID;

SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
ON TABLE_A.ID = TABLE_B.ID
left JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

Carlo Romagnano SSC-Insane Points: 22599 More actions · Answer 7

ChrisM@Work - Wednesday, November 7, 2018 3:36 AM
Force the join between B and C to take place first, and you change the result:
SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
INNER JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID
ON TABLE_A.ID = TABLE_B.ID;
-- 3 rows

Thanks! That is what I'm searching for!

ChrisM@Work SSC Guru Points: 186127 More actions · Answer 8

Carlo Romagnano - Wednesday, November 7, 2018 4:32 AM
ChrisM@Work - Wednesday, November 7, 2018 3:36 AM
Force the join between B and C to take place first, and you change the result:
SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
INNER JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID
ON TABLE_A.ID = TABLE_B.ID;
-- 3 rows
Thanks! That is what I'm searching for!

You're welcome. There's a little more info and a better example here.

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

Shayn Thomas SSCertifiable Points: 5863 More actions · Answer 9

nice and simple,

ta

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The more you know, the more you know that you dont know

call.copse SSCoach Points: 17958 More actions · Answer 10

ChrisM@Work - Wednesday, November 7, 2018 3:57 AM
call.copse - Wednesday, November 7, 2018 3:50 AM
ChrisM@Work - Wednesday, November 7, 2018 3:36 AM
Force the join between B and C to take place first, and you change the result:
SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
INNER JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID
ON TABLE_A.ID = TABLE_B.ID;
-- 3 rows
I get what you're saying, but wouldn't it be clearer to left join to C?
No, because the queries are not logically equivalent:
SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
INNER JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID
ON TABLE_A.ID = TABLE_B.ID;
SELECT *
FROM TABLE_A
LEFT JOIN TABLE_B
ON TABLE_A.ID = TABLE_B.ID
left JOIN TABLE_C
ON TABLE_B.ID = TABLE_C.ID

I read the link you posted below and now consider myself elucidated, thank you. After all these years there is plenty to learn even on relatively basic querying!

john.lynn-871155 SSC-Addicted Points: 448 More actions · Answer 11

john.lynn-871155

SSC-Addicted

Points: 448

November 8, 2018 at 4:23 am

#2012792

Nice one Ben..