單行子查詢
結果集只有一行
一般搭配單行操作符使用：> < = <> >= <=
非法使用子查詢的情況：

多行子查詢
結果集有多行
一般搭配多行操作符使用：any、all、in、not in
in：屬于子查詢結果中的任意一個就行
any和all:往往可以用其他查詢代替

誰的工資比abel高?
1查詢Abel的工資
SELECT salary
FROM employees
WHERE last_name = 'Abel'

2查詢員工的信息，滿足 salary>1結果
SELECT *
FROM employees
WHERE salary>(
SELECT salary
FROM employees
WHERE last_name = 'Abel'
);
把1的結果放在2的小括號內,括號內就是子查詢。

查詢公司的 最低工資
SELECT MIN(salary)
FROM employees

查詢last_name,job_id和salary，要求 salary=1
SELECT last_name,job_id,salary
FROM employees
WHERE salary=
(
SELECT MIN(salary)
FROM employees
);

查詢最低工資大于50號部門最低工資的部門id和其最低工資

查詢50號部門的最低工資
SELECT MIN(salary)
FROM employees
WHERE department_id = 50

查詢每個部門的最低工資

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id

在②基礎上篩選，滿足min(salary)>①
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>
(
SELECT MIN(salary)
FROM employees
WHERE department_id = 50
);

返回location_id是1400或1700的部門中的所有員工姓名

查詢location_id是1400或1700的部門編號
SELECT distinct department_id
FROM departments
WHERE location_id IN(1400,1700)

查詢員工姓名，要求部門號是①列表中的某一個
SELECT last_name
FROM employees
WHERE department_id in
(
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700)
);

返回其它工種中比job_id為‘IT_PROG’工種任一工資低的員工的員工號、姓名、job_id 以及salary

查詢job_id為‘IT_PROG’部門任一工資
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'

查詢員工號、姓名、job_id 以及salary，salary<(①)的任意一個
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary < ANY
(
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'

) AND job_id <> 'IT_PROG';
或(< any等價于< min)
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary <
(
SELECT min(salary)
FROM employees
WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

查詢最小的員工編號
SELECT MIN(employee_id)
FROM employees

查詢最高工資
SELECT MAX(salary)
FROM employees

查詢員工信息
SELECT *
FROM employees
WHERE employee_id=
(
SELECT MIN(employee_id)
FROM employees
)
AND salary=
(
SELECT MAX(salary)
FROM employees
);

查詢每個部門的平均工資
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
然后再
SELECT * FROM job_grades;
顯示查詢結果

連接①的結果集和job_grades表，篩選條件平均工資 between lowest_sal and highest_sal
SELECT ag_dep.*,g.'grade_level'
FROM
(
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id
) as ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;

• 用in
  SELECT department_name
  FROM departments d
  WHERE d.'department_id' IN
  (
  SELECT department_id
  FROM employees
  );
• 用exists
  SELECT department_name
  FROM departments d
  WHERE EXISTS
  (
  SELECT *
  FROM employees e
  WHERE d.'department_id'=e.'department_id'
  );