1

Assume that we have two tables in SQL database (I'm using PostgreSQL in case the answer depends on the database):

CREATE TABLE testA
(
    id integer,
    checked boolean
)

CREATE TABLE testB
(
    id integer,
    testA_id integer
)

I want to do a select from testB joined with testA and get all results which are checked. There are two ways:

SELECT tA.*, tB.*
FROM testB AS tB
JOIN testA AS tA
    ON (tA.id = tB.testA_id AND tA.checked)

or 

SELECT tA.*, tB.*
FROM testB AS tB
JOIN testA AS tA
    ON tA.id = tB.testA_id
WHERE
    tA.checked

Which way is preferred? And are there performance differences?

4

2 回答 2

4

Well, the WHERE clause filters the results. The ON just defines the way the tables are joined together. In your example there would be no difference.

But take a look at the following:

CREATE TABLE testA
(
    id integer,
    checked boolean
)

CREATE TABLE testB
(
    id integer,
    testA_id integer,
    checked boolean
)

As you can see i have added a checked column in TestB.

There might be rows in TableA that have no rows in TableB Like the following data:

TestA

ID|checked
1|true
2|true
3|false

TestB

ID|testA_id|checked
1|1|true
2|1|false
3|3|true

As you can see there is no TestB for TestA id = 2

Now, let's assume you want to display ALL TestA rows along with the checked TableB rows (if any).

So you need a left join:

First Case

SELECT tA.*, tB.*
FROM testB AS tB
LEFT JOIN testA AS tA
    ON (tA.id = tB.testA_id AND tB.checked)

Reults

ID|checked|ID|testA_id|checked
1|true|1|1|true
2|true|null|null|null
3|false||3|3|true

ID 1, 2 and 3 of TableA is returned and if there is a checked TableB row then we return that too.

Second Case

SELECT tA.*, tB.*
FROM testB AS tB
LEFT JOIN testA AS tA
    ON tA.id = tB.testA_id
WHERE
    tB.checked

Reults

ID|checked|ID|testA_id|checked
1|true|1|1|true
3|false||3|3|true

In this case only IDs 1 and 3 are returned because we filter the results to show only the ones that have TableB checked.

于 2013-08-23T09:45:36.210 回答
1

No performance differences, the optimizer takes care of it, so the two queries are actually the same. You can confirm by using EXPLAIN on your query.

于 2013-08-23T09:29:15.023 回答