Introduction and advanced level of MySQL (10)

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1、SQL Intersection and difference operation of

sql The result set of the query in can be understood as ⼀ A collection of
MySql The database only ⽀ Union set
union: Merge result set , Meeting ⾃ Dynamic weight removal , Repeat only keep ⼀ individual .ABCDEF
union all ： Merge result set , But it will not rule out repetition .ABCCDEF
unionall efficiency ⽐union⾼
Eg： List the schools in Zhengzhou and Luoyang ⽣ Information

 List the student information of Zhengzhou and Luoyang 
select * FROM studentinfo 3 where city=' luoyang ' or city=' zhengzhou ' 4 -- union Merger premise ： Same number of columns , Types can be compatible 
select * FROM studentinfo
where city=' luoyang ' 7 union all 8 select * FROM studentinfo 9 where city=' zhengzhou ' 1011 --  Merge of result sets , As long as the number of columns is the same , Types can be merged if they are compatible  1213 select classid,classname from class 14 union all 15 select studentno,studentname from studentinfo

2、 The concept of subquery

⼦ Query is ⼀ Nested in SELECT、INSERT、UPDATE or DELETE Query in statement
The database engine will ⼦ Query is executed as a virtual table ⾏ Query operation .
⼦ Queries can be used as ⼀ Tables , It can also be used as ⼀ It's worth .
⼦ Query execution ⾏ Rely on nested queries . The sequence starts from the innermost layer ,⼀ layer ⼀ Layer outward ⾏, The outer nested query can access the results of the inner nested query , phase ⽐ Variable ⽅ Hold on to ⾏ More efficient ⾼,
⼦ Queries can also combine data from multiple tables into ⼀ rise .
⼦ Inquire about ：（ Will retrieve —— Single table 、 Multiple tables 、 polymerization 、⼦ Inquire about ） As a result ⼀ Virtual table data participates in the operation of other statements ⾏.
Can exist as a table , It can also exist as a value ⼦ Query is ok ⽤ Association query implementation , however ⼀ Look at it ⼦ Query to ⽐ Relational query ⾼; limitations ： Such as ⼦ The query exists as a table ——select Later change can ⽤⼦ Columns to query ; If ⼦ Query as retrieved value set The value of , Can't be in select In the column name position of ⼦ The column name of the query .