Mysql how to join tables [closed]

问题:

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Closed 3 years ago.

I have three tables:

• Table 1 contains a primary key.
• Table 2 contains a foreign key that is equal to primary key of table 1.
• Table 3 contains a foreign key that is equal to primary key of table 2.

QUESTION: Is it possible to SELECT information from table 2 and 3 while only knowing the primary key of table 1? If so please give a brief example of the SELECT QUERY. I have tried doing an Inner join but got a syntax error.

回答1:

Yes you can.

Example:

table_a              table_b             table_c
 _______________      _______________     _______________
|  id  |  name  |    |  id  | gender |   |  id  |   age  |
|------+--------|    |------+--------|   |------+--------|
|   1  |  sam   |    |   1  |    m   |   |   1  |   18   |
|------+--------|    |------+--------|   |------+--------|
|   2  |  ana   |    |   2  |    f   |   |   2  |   22   |
|------+--------|    |------+--------|   |------+--------|

In order to get the following result:

 _________________________________ 
|  id  |  name  | gender |   age  |
|------+--------+--------+--------|
|   1  |  sam   |    m   |   18   |
|------+--------+--------+--------|
|   2  |  ana   |    f   |   22   |

You could use the following SQL statement:

SELECT a.id, a.name, b.gender, c.age
FROM table_a AS a
LEFT JOIN table_b AS b
    ON a.id = b.id
LEFT JOIN table_c AS c
    ON a.id = c.id

P.S.: only answered this to do the ascii art xD!