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What is the multi-table joint query statement in mysql

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2020-09-03 14:01:2511466browse

The multi-table union query statement in mysql is: [select statement 1 union [union option] select statement 2 union [union option] select statement n]. Multi-table joint query results combine the query results of multiple select statements together.

What is the multi-table joint query statement in mysql

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MySQL multi-table joint query statement is:

The joint query result is to combine the query results of multiple select statements together.

You can use the union and union all keywords to merge.

Basic syntax:

select statement 1

union [union option]

select statement 2

union [union Options]

select statement n

The union option has two options: all (indicates duplicates are also output); distinct (removes duplicates, completely duplicates, will be removed by default) Heavy)

The fields of the two tables can be consistent.

例:
select id,addrid 
from addr 
union all 
select id,addrid 
from student

The meaning of joint query

1. Query the same table, but with different requirements

2.Multiple table query: the structure of multiple tables Exactly the same, the saved data (structure) is also the same

Use of order by in joint query

In joint query: order by can only be used at the end, and you need to use parentheses for the query statement. OK.

例:
---(错误)
select * from student where sex="man" order by score
union
select * from student wherre sex="woman" order by score;
这种情况会报错,因为一个句子中不能有两个order by
---(正确但不符合所需)
select * from student where sex="man" 
union
select * from student wherre sex="woman" order by score;
这种情况是正确的,但是合并又没有意义,他会把之前的sex分好的情况给打乱
---(正确)
(select * from student where sex="man" order by score 
limit 10)
union
(select * from student wherre sex="woman" order by score
limit 10);
在子语句中使用order by,由于优先级的问题,需要将整个子句用()括起来,且必须和limit结合使用,否则不会生效。

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