Usage of exists in sql

Existing: Class Table (A_CLASS)

STUDENT

Note: The classId of the student table (STUDENT) is associated with the primary key ID of the class table (A_CLASS)

Code:

select * from STUDENT s WHERE exists (select 1 from A_ClASS c where s.CLASS_ID=c.ID)

Results

[The execution order of the exists statement is as follows]:

**1.**The outer loop (select * from student) will be executed first

**2.**Each row of the results returned by the outer loop will be taken to the inner loop for execution (at this time, note that the inner layer is also cyclically queried)

ps: The query of select * from student is (c1,c1,c2,c3); at this time, the execution order should be (

The first time: select* from A_CLASS where c1=c1, the second time: select* from A_CLASS where c1=c2,

The third time: select* from A_CLASS where c1=c1, the fourth time: select* from A_CLASS where c1=c2,

The fifth time: select* from A_CLASS where c2=c1, the sixth time: select* from A_CLASS where c2=c2,

The seventh time: select* from A_CLASS where c3=c1, the eighth time: select* from A_CLASS where c3=c2)

Note: If the inner subquery is true at this time, it will return directly and will not continue to execute this loop;

In summary: the second and fourth times will not be executed; the first and third and sixth times are eligible;

So the data whose CLASS_ID field in the STUDENT table is (c1,c2) ​​will be queried;

If you replace the = in the above statement with != What will be the effect?

select * from STUDENT s WHERE exists (select 1 from A_ClASS c where s.CLASS_ID !=c.ID)

If the result looks like this:

Specific analysis: The execution at this time should be exactly the same as the first time, and it is also cycled 8 times:

First time: select* from A_CLASS where c1!=c1, the second time: select* from A_CLASS where c1!=c2,

Third time: select* from A_CLASS where c1!=c1, the fourth time: select* from A_CLASS where c1!=c2,

Fifth time: select* from A_CLASS where c2!=c1, sixth time: select* from A_CLASS where c2!=c2,

Seventh time: select* from A_CLASS where c3!=c1, the eighth time: select* from A_CLASS where c3!=c2

But = replaced!=; This is the sixth and eighth times not executed, the rest are all executed, s.CLASS_ID involves (c1, c2, c3) and the statements all return true;

So the data whose CLASS_ID field in the STUDENT table is (c1,c2,c3) will be queried;

Summary:

1.exists will take the value of the outer loop to query the inner layer after executing the outer loop. If the query is found, it will directly return true and terminate the current loop. If it is false, it will continue to execute until the loop is completed.If it is still false, the inner loop does not meet the conditions;

2. Do not write the inner judgment conditions!=;The result of the query will be unsatisfactory;