MySQL -- subquery scalar subquery

Scalar subquery

MySQL --- Subquery - Sub query concept 、 standard 、 classification

The sub query returns the data of single row and single column , It's a value

Mainly used in WHERE clause

You need to use some operators Greater than ＞、 Less than ＜、 be equal to ＝、 It's not equal to ＜＞、！＝

Writing method of subquery

① Write from outside to inside

② Write from the inside out

• Find the basic salary ratio ALLEN Low total employee information

Write from outside to inside

--  Find out all employee information 
SELECT * FROM emp;
-- ALLEN The salary of 
SELECT sal FROM emp WHERE ename='ALLEN';
--  Query out ([ Basic wage ratio ALLEN Low ] All employee information )
SELECT * FROM emp WHERE sal<(SELECT sal FROM emp WHERE ename='ALLEN');

 

• Query the information of all employees whose basic salary is higher than the average salary of the company

--  Query the average salary of the company 
SELECT AVG(sal) FROM emp;/*1988.3333*/
--  Query the information of all employees whose basic salary is higher than the average salary of the company 
SELECT * FROM emp WHERE sal>1988.3333;
--  Inquire about [ The basic salary is higher than ( Average salary of the company ) All employee information ]
SELECT * FROM emp WHERE sal>(SELECT AVG(sal) FROM emp);

Write from the inside out

MySQL --- Database query - The use of aggregate functions 、 Aggregate query 、 Group query

• Find out and ALLEN In the same job , And the basic salary is higher than the employee number 7521 All employee information

--  Find out and ALLEN In the same job  
SELECT * FROM emp
WHERE job=(SELECT job FROM emp WHERE ename='ALLEN')
--  And the basic salary is higher than the employee number 7521 All employee information 
AND sal>(SELECT sal FROM emp WHERE empno=7521)
--  hold  ALLEN  Remove from query results 
AND ename<>'ALLEN';

 

You can find that the query results contain ALLEN, Need to put ALLEN Remove from query results

Single line sub query

Subquery returns single row and multi column data , It's just a record

• Query and SCOTT Information of employees who are engaged in the same job and have the same salary

--  Query and SCOTT Information of employees who are engaged in the same job and have the same salary 
SELECT * FROM emp
WHERE job=(SELECT job FROM emp WHERE ename='SCOTT')
AND sal=(SELECT sal FROM emp WHERE ename='SCOTT')
AND ename<>'SCOTT';

Use scalar subqueries

Use a single line subquery

--  Inquire about SCOTT Employee information 
SELECT job,sal FROM emp WHERE ename='SCOTT';
--  Query and SCOTT Information of employees who are engaged in the same job and have the same salary 
SELECT * FROM emp WHERE (job,sal)=(SELECT job,sal FROM emp WHERE ename='SCOTT');

• The inquiry and employee number are 7566 Information of all employees who are engaged in the same work and lead the same

--  The inquiry and employee number are 7566 Information of all employees who are engaged in the same work and lead the same 
SELECT * FROM emp WHERE 
(job,mgr)=(SELECT job,mgr FROM emp WHERE empno=7566);

• Query and ALLEN Information of all employees engaged in the same job and employed in the same year ( contain ALLEN)

Same number of columns 、 The same data type can be compared directly

MySQL --- Common functions - Numerical function 、 Time date function 、 Conditional judgment function 、 Other functions

--  Query and ALLEN Information of all employees engaged in the same job and employed in the same year ( contain ALLEN)
SELECT * FROM emp WHERE
(job,DATE_FORMAT(hiredate,'%Y'))=
(SELECT job,DATE_FORMAT(hiredate,'%Y') FROM emp WHERE ename='ALLEN');