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.
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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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