MySQL子查詢

1、什么是子查詢

　　嵌套在其它語句內部的select語句稱為子查詢(也稱為內查詢),這里面的其它語句可以是insert、delete、update、select,一般情況下select語句作為外部語句居多,如果外部的語句為select語句,則將外部的select語句稱為主查詢(也稱為外查詢).

 

2、子查詢特點

　　1、子查詢優於主查詢執行,主查詢使用子查詢的執行結果.

　　2、子查詢都放在小括號內.

　　3、子查詢可以放在 from、select、where、having、exists后面,但是一般是放在查詢條件的右側.

 

3、子查詢的分類　　

　　按照查詢的結果分類

　　　　1、標量子查詢(單行子查詢):結果集為一行一列.

　　　　2、列子查詢(多行子查詢):結果集為多行一列.

　　　　3、行子查詢:結果集為一行多列.

　　　　4、表子查詢:結果集為多行多列.

　　

4、子查詢案列　

　　表結構如下:

                              

 

　　1、誰的工資比Chen高.

SELECT last_name,salary FROM employees WHERE salary > ( SELECT salary FROM employees WHERE last_name = 'chen' );

 

　　2、查詢job_id與141號員工相同,salary比143號員工多的員工id、員工姓名、job_id、salary.

SELECT 
　　 employee_id,
　　 last_name,
　　 job_id,
　　 salary FROM employees WHERE job_id = ( SELECT job_id FROM employees WHERE employee_id = 141 ) AND salary > ( SELECT salary FROM employees WHERE employee_id = 143 );

 

　　3、查詢公司工資最少的員工的last_name,job_id和salary.

SELECT last_name,job_id,salary
FROM employees
WHERE salary = (
    SELECT MIN(salary)
    FROM employees
);

 

　　4、查詢最低工資大於50號部門最低工資的部門id和其最低工資.

SELECT department_id, MIN(salary) FROM employees GROUP BY department_id HAVING MIN(salary) > ( SELECT　MIN(salary) FROM employees WHERE　department_id = 50 );

 

　　5、查詢location_id是1400、1700或者1800的部門中的所有員工姓名.

SELECT last_name, department_id FROM employees WHERE department_id IN ( SELECT department_id FROM departments WHERE location_id IN (1400,1700,1800) ); 

 

　　6、查詢其它工種中比job_id為IT_PROG工種任意工資低的員工的員工號、姓名、job_id 以及salary.

SELECT employee_id, last_name, job_id, salary FROM employees WHERE salary < ( SELECT MAX(salary) FROM employees WHERE job_id = 'IT_PROG' ) AND job_id != 'IT_PROG';

 

　　7、查詢和Zlotkey相同部門的員工姓名和工資.

SELECT last_name, salary FROM employees WHERE department_id = ( SELECT department_id FROM employees WHERE last_name = 'Zlotkey' );

 

　　8、查詢工資比公司平均工資高的員工的員工號、姓名和工資.

SELECT employee_id, last_name, salary FROM employees WHERE salary > ( SELECT AVG(salary) FROM employees );

 

　　9、查詢各部門中工資比本部門平均工資高的員工的員工號、姓名和工資,並按照員工號升序排序.

SELECT e.employee_id, e.last_name, e.salary, avg_dep.ag FROM employees e INNER JOIN( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ) AS avg_dep ON e.department_id = avg_dep.department_id WHERE e.salary > avg_dep.ag ORDER BY e.employee_id ASC;

 

　　10、查詢和姓名中包含字母u的員工在相同部門的員工的員工號和姓名.

SELECT e.employee_id, e.last_name FROM employees e WHERE e.department_id IN ( SELECT DISTINCT department_id FROM employees 　　 WHERE last_name LIKE '%u%' ) AND e.last_name NOT LIKE '%u%';    

　　

　　11、查詢在部門的location_id為1700的部門工作的員工號和員工名.

SELECT e.employee_id, e.last_name FROM employees e WHERE department_id IN( ( 　　SELECT d.department_id 　　FROM departments d 　　WHERE d.location_id = 1700 ) );