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MySQL实例讲解子查询的使用

瀛台夜雪 人气:0

子查询-嵌套查询

子查询是指一个查询语句嵌套在另一个语句内部的查询

原始查询方法

SELECT last_name,salaryFROM employeesWHERE last_name='Abel';SELECT last_name,salaryFROM employeesWHERE salary>11000;

自连接

SELECT e2.last_name,e2.salaryFROM employees e1,employees e2WHERE e2.salary>e1.salary AND e1.last_name='Abel';

子查询

SELECT last_name,salaryFROM employeesWHERE salary>(SELECT salaryFROM employeesWHERE last_name='Abel');

称谓的规范:外查询(或主查询),内查询

注意事项

子查询分类

单行子查询 vs 多行子查询

相关子查询vs不相关子查询

单行子查询

单行比较运算符

操作符含义
=equal to
>greater than
>=greater than or equal to
<less than
<=less than or equal to
<>not equal to

子查询的编写思路

SELECT * 
FROM employees
WHERE salary>(
		SELECT salary
		FROM employees
		WHERE employee_id=149
);
SELECT 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
);
SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id=(
									SELECT manager_id
									FROM employees
									WHERE employee_id=141
)
AND 
department_id=(
									SELECT department_id
									FROM employees
									WHERE employee_id=141
)
AND
employee_id<>141;
SELECT employee_id,manager_id,department_id
FROM employees
WHERE (manager_id,department_id)=(
									SELECT manager_id,department_id
									FROM employees
									WHERE employee_id=141
)
AND 
employee_id<>141;

HAVING中的子查询

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

CASE中的子查询

在CASE表达式中使用单列子查询

SELECT employee_id,last_name ,CASE department_id WHEN (SELECT department_id FROM departments WHERE location_id=1800) THEN 'Canada'																								ELSE 'USA' END 'location'
FROM employees;

子查询中的空值问题

若内查询查询结果为空的,则不会报错,子查询不会返回任何行

非法使用子查询

多行子查询

多行子查询也被称为集合比较子查询

内查询返回多行数据

使用多行比较操作符

多行比较操作符

操作符含义
IN等于列表中的任意一个
ANY需要和单行比较操作符一起使用,和子查询返回的某一个值比较(任一)
ALL需要和单行比较操作符一起使用,和子查询返回的所有值比较
SOME实际上是ANY的别名,作用相同,一般常使用ANY

MySQL中聚合函数是不能嵌套的

SELECT employee_id,last_name
FROM employees 
WHERE salary IN
(
	SELECT MIN(salary)
	FROM employees
	GROUP BY department_id
);
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE job_id<>'IT_PROG'
AND salary < ANY (
	SELECT salary 
	FROM employees
	WHERE job_id='IT_PROG'
);
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE job_id<>'IT_PROG'
AND salary < ALL (
	SELECT salary 
	FROM employees
	WHERE job_id='IT_PROG'
);
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL (
	SELECT AVG(salary) avgsal
	FROM employees
	GROUP BY department_id
);
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary)=
(
SELECT MIN(avgsal)
FROM (
SELECT AVG(salary) avgsal
FROM employees
GROUP BY department_id
) dbsal
);

多行子查询空值问题

子查询中存在空值的情况将使得查询结果生成的位空

相关子查询

如果子查询的执行依赖于外部查询,通常情况下是因为子查询中的表用到了外部的表,并进行了条件关联,因此每职系那个一次外部查询,子查询都需要重新计算一次,这样的子查询便被称之为关联子查询

相关子查询按照一行接一行的顺序执行,主查询的每一行都执行一次子查询

SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
	SELECT AVG(salary) 
	FROM employees e2 
	WHERE department_id=e1.department_id
);
#在from中进行子查询
SELECT e1.last_name,e1.salary,e1.department_id
FROM employees e1,
(
SELECT department_id ,AVG(salary) avgs
FROM employees
GROUP BY department_id
) t1
WHERE e1.department_id=t1.depa
SELECT employee_id,salary
FROM employees e
ORDER BY(
	SELECT department_name
	FROM departments d
	WHERE e.department_id=d.department_id
) ASC;
SELECT employee_id,last_name,job_id
FROM employees e
WHERE 2<= (
	SELECT COUNT(*)
	FROM job_history j
	WHERE e.employee_id=j.employee_id
)

结论:在SELECT中,除了GROUP BY和LIMIT之外,其他位置都可以声明子查询

EXISTS与NOT EXISTS 关键字

关联子查询通常也会和EXISTS操作符一起来使用,用来检查在子查询中是否存在满足条件的行

如果在子查询中不存在满足条件的行

如果在子查询中存在满足条件的行

NOT EXISTS关键字表示不存在某种条件,则返回TRUE,否则返回FALSE

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