Introduction and advanced MySQL (XIV)

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Query result set as ⼀ Virtual tables directly participate in the query

Update send ⽤⼦ Inquire about ： Can appear in set The position of the value can also appear where Conditional location
Eg： Those who fail will learn ⽣ Class changed to id most ⼤ In my class
Delete send ⽤⼦ Inquire about ： Only in conditional positions
Eg： Delete all the score information of the lowest score
Eg： Delete all the score information of the lowest score :where Conditions can be written in subqueries , Modified delete query , however ：mysql It does not support deleting or modifying a table while querying , Nesting a subquery can solve .

 delete from exam where exam=(
 select * from(select min(exam) from exam)a
 -- Eg： Change the failing students' class to id In the largest class 
 --  Subqueries can be used as search conditions or as values 
 select max(classid) from class
select studentid from exam where exam<60
update studentinfo set classid=(select max(classid) from class)
 where studentid in(
select studentid from exam where exam<60
 )

adopt EXISTS lead ⼊ The existence of the test ⼦ Inquire about
with EXISTS The predicate ⼦ The query does not return any actual data
Only produce ⽣ Logical truth “true” Or logical false value “false
Eg： Inquire about C# The course exam is just equal to 60 Divided learning ⽣ list
EXISTS: His result set value is useless , Just judge whether there is a result set , If there is a result set, the peripheral query results, otherwise the peripheral query of this line will not be displayed .
Judge whether the result set has value , promising true No, false.
Execution order ： The peripheral query of each row will judge the sub query inside , If the subquery has a result of true Not for false
If there are many peripheral queries , There are few sub queries exits It doesn't make much sense
If there is less peripheral data , There are many sub queries , here exits It's a lot more efficient
exitts Internal and external inquiries must be connected .
Eg： Inquire about C# The course exam is just equal to 60 A list of students

select subjectid from `subject` where subjectname=' be based on C Language understanding software 
 Programming '

 select * from exam where exam=60 and subjectid=1
select * from studentinfo where studentid in(
select studentid from exam where exam=60 and subjectid=1
 )
 --  Students in every line , Use both id Go to exam Query in table , If the return value is recorded , So stupid students stay or don't show 
 select * from studentinfo a where EXISTS(
select * from exam b where exam=60 and subjectid=1 and
a.studentid =b.studentid)