exists用法

exists的用法

牛客网刷题：SQL57 使用含有关键字exists查找未分配具体部门的员工的所有信息
SQL57

用法1：建表时使用

用于判断表是否存在

drop table if exists employees;
drop table if exists dept_emp;
CREATE TABLE `employees` (
  `emp_no` int(11) NOT NULL,
  `birth_date` date NOT NULL,
  `first_name` varchar(14) NOT NULL,
  `last_name` varchar(16) NOT NULL,
  `gender` char(1) NOT NULL,
  `hire_date` date NOT NULL,
  PRIMARY KEY (`emp_no`));
CREATE TABLE `dept_emp` (
`emp_no` int(11) NOT NULL,
`dept_no` char(4) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`dept_no`));
INSERT INTO employees VALUES(10001,'1953-09-02','Georgi','Facello','M','1986-06-26');
INSERT INTO employees VALUES(10002,'1964-06-02','Bezalel','Simmel','F','1985-11-21');
INSERT INTO employees VALUES(10003,'1959-12-03','Parto','Bamford','M','1986-08-28');
INSERT INTO employees VALUES(10004,'1954-05-01','Chirstian','Koblick','M','1986-12-01');
INSERT INTO employees VALUES(10005,'1955-01-21','Kyoichi','Maliniak','M','1989-09-12');
INSERT INTO employees VALUES(10006,'1953-04-20','Anneke','Preusig','F','1989-06-02');
INSERT INTO employees VALUES(10007,'1957-05-23','Tzvetan','Zielinski','F','1989-02-10');
INSERT INTO employees VALUES(10008,'1958-02-19','Saniya','Kalloufi','M','1994-09-15');
INSERT INTO employees VALUES(10009,'1952-04-19','Sumant','Peac','F','1985-02-18');
INSERT INTO employees VALUES(10010,'1963-06-01','Duangkaew','Piveteau','F','1989-08-24');
INSERT INTO employees VALUES(10011,'1953-11-07','Mary','Sluis','F','1990-01-22');
INSERT INTO dept_emp VALUES(10001,'d001','1986-06-26','9999-01-01');
INSERT INTO dept_emp VALUES(10002,'d001','1996-08-03','9999-01-01');
INSERT INTO dept_emp VALUES(10003,'d004','1995-12-03','9999-01-01');
INSERT INTO dept_emp VALUES(10004,'d004','1986-12-01','9999-01-01');
INSERT INTO dept_emp VALUES(10005,'d003','1989-09-12','9999-01-01');
INSERT INTO dept_emp VALUES(10006,'d002','1990-08-05','9999-01-01');
INSERT INTO dept_emp VALUES(10007,'d005','1989-02-10','9999-01-01');
INSERT INTO dept_emp VALUES(10008,'d005','1998-03-11','2000-07-31');
INSERT INTO dept_emp VALUES(10009,'d006','1985-02-18','9999-01-01');
INSERT INTO dept_emp VALUES(10010,'d005','1996-11-24','2000-06-26');
INSERT INTO dept_emp VALUES(10010,'d006','2000-06-26','9999-01-01');

用法2：查询时使用

放在where后面，格式如下

select ... where exists (subquery);

• 如果子查询有任意数据返回，exists就返回true，子查询外的查询语句就执行
• 如果子查询没有数据返回（数据为空），exist就返回false，子查询外的查询语句就不执行

简单的进行一下测试

SELECT emp_no FROM dept_emp WHERE emp_no>10000;

SELECT * FROM employees WHERE EXISTS(SELECT emp_no FROM dept_emp WHERE emp_no>10000);

SELECT emp_no FROM dept_emp WHERE emp_no>10005;

SELECT * FROM employees WHERE EXISTS(SELECT emp_no FROM dept_emp WHERE emp_no>10005);

从上面的结果我们可以看出，只要exists后面的子查询中存在结果，有返回值，其查询结果是一样的。

SELECT * FROM employees WHERE EXISTS(SELECT emp_no FROM dept_emp WHERE emp_no<10000);

当子查询没有数据返回的时候，整个查询结果就位空，也就是子查询外的查询语句未执行。

SQL57题的解法

select *
from employees
where not exists
(
    select emp_no
    from dept_emp
    where dept_emp.emp_no = employees.emp_no
);

也可以用IN来实现。