目录子查询-嵌套查询原始查询方法自连接子查询子查询分类单行子查询子查询的编写思路HAVING中的子查询CASE中的子查询子查询中的空值问题多行子查询多行比较操作符相关子查询EXISTS与NOT EXISTS 关键
子查询是指一个查询语句嵌套在另一个语句内部的查询
SELECT last_name,salaryFROM employeesWHERE last_name='Abel';SELECT last_name,salaryFROM employeesWHERE salary>11000;
SELECT e2.last_name,e2.salaryFROM employees e1,employees e2WHERE e2.salary>e1.salary AND e1.last_name='Abel';
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
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 |
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;
SELECT department_id,MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
SELECT MIN(salary)
FROM employees
WHERE department_id=50
);
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;
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
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
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
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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