问题
I have 2 tables in T-SQL one with 10 records and one with 11.
select tbl.unit, tbl2.unid
from tbl1
left join tbl2 on tbl2.UNID = tbl1.UNIT
where tbl2.Status = 'Main'
group by unit, UNID
This only returns 10 records when tbl1 has 11 records. I was expecting the missing record to show a value if UNIT but null for UNID but is just is not there.
I cannot figure out why this is the case
回答1:
So why would a LEFT JOIN not show all the records from a left side of the join.
Is a bug?
Most likely not.
Lets look at a simplified example.
TableA has 3 records.
ID ColA
1 Foo
2 Bar
3 Buz
TableB has 2 records
ID ColB
4 Foo
5 Bar
An INNER JOIN on ColA & ColB would return 2 records.
Only those where a match is found.
SELECT ColA, ColB
FROM TableA a
JOIN TableB b ON b.ColB = a.ColA
Returns:
ColA ColB
Foo Foo
Bar Bar
A LEFT JOIN would return 3 records.
With a NULL for the right side for the unmatched.
SELECT ColA, ColB
FROM TableA a
LEFT JOIN TableB b ON b.ColB = a.ColA
Returns:
ColA ColB
Foo Foo
Bar Bar
Buz null
But what happens if a criteria is used in the WHERE clause for the right side?
SELECT ColA, ColB
FROM TableA a
LEFT JOIN TableB b ON b.ColB = a.ColA
WHERE b.ColB IN ('Foo', 'Bar', 'Buz')
Returns:
ColA ColB
Foo Foo
Bar Bar
What? Where is the 'Buz'?
Can you guess why that LEFT JOIN seems to behave like an INNER JOIN?
The solution is to put such criteria in the ON clause.
SELECT ColA, ColB
FROM TableA a
LEFT JOIN TableB b
ON b.ColB = a.ColA AND b.ColB IN ('Foo', 'Bar', 'Buz')
Or do put the criteria in the WHERE, but also allow NULL.
SELECT ColA, ColB
FROM TableA a
LEFT JOIN TableB b
ON b.ColB = a.ColA
WHERE (b.ColB IN ('Foo', 'Bar', 'Buz') OR b.ColB IS NULL)
Returns:
ColA ColB
Foo Foo
Bar Bar
Buz null
Now we have Buz back.
来源：https://stackoverflow.com/questions/59038630/t-sql-left-join-not-returning-null-columns