The difference between union and union all in MySQL

One 、 difference 1： Take the intersection of results

1、union: Combine two result sets , Do not include repeating lines , amount to distinct, At the same time, sort the default rules ;

2、union all: Combine two result sets , Include repeating lines , That is, all the results are displayed , Whether it's repetition or not ;

Two 、 difference 2： The operation after getting the result

1、union: Will sort the obtained results

2、union all: No sort operation will be performed on the obtained results

3、 ... and 、 difference 3：

1、union See the results ID=3 There is only one

select * from student2 where id < 4
union
select * from student2 where id > 2 and id < 6

2、union all In the end ID=3 There are two results

select * from student2 where id < 4
union all
select * from student2 where id > 2 and id < 6

Four 、 summary

union all Just merge query results , There's no de duplication or sorting , On the premise of not removing the weight , Use union all It's more efficient than union high

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