Using MySQL 5.x I want to efficiently select all rows from table X where开发者_StackOverflow there is no related row in table Y satisfying some condition, e.g.
Give me all records in X where a related Y with foo = bar does NOT exist
SELECT count(id) FROM X
LEFT OUTER JOIN Y ON y.X_id = X.id AND y.foo = 'bar'
WHERE y....?
As I understand it, a left outer join is guaranteed to produce a row for each row in the left (first) table -- X in this case -- whether or not a satisfying row in the joined table was found. What I want to do is then select only those rows where no row was found.
It seems to me that y.X_id should be NULL if there is no matching record, but this test doesn't seem to work. Nor does y.X_id = 0 or !y.X_id.
Edits: corrected transcription error (ON not AS) which was pointed out by several responses. Fixed grammatical error.
SELECT count(id) FROM X
LEFT OUTER JOIN Y ON (y.X_id = X.id AND y.foo = 'bar')
WHERE y.X_id is null
You were close.
First do the join as normal, then select all rows for which a not null
row in Y is in fact null
, so you are sure there's a "no match" and not just a null
value in Y.
Also note the typo (since corrected) you made in the query:
LEFT OUTER JOIN Y AS
-- should be
LEFT OUTER JOIN Y ON
-- This however is allowed
LEFT OUTER JOIN table2 as Y ON ....
Checking if the primary key of table Y is NULL would do the trick, which tells the join did not matched :
SELECT count(id) FROM X
LEFT OUTER JOIN Y ON (y.X_id = X.id AND y.foo = 'bar')
WHERE y.Y_id is null
Johan's answer is correct 100%.
Besides that, there is also this option:
SELECT count(id)
FROM X
WHERE NOT EXISTS
( SELECT *
FROM Y
WHERE (y.X_id = X.id AND y.foo = 'bar')
)
Depending on your table size and data distribution, this may be more efficient. Test and keep both ways for future reference.
Why use an outer join? Couldn't you just do:
SELECT count(id)
FROM X JOIN Y AS y.X_id = X.id AND y.foo <> 'bar'
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