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SELECT SUM(salary) AS 和,AVG(salary) AS 平均,MAX(salary) AS 最大,MIN(salary) AS 最小,COUNT(salary) AS 數(shù)量
FROM employees;

SELECT SUM(DISTINCT salary) AS 和,AVG(DISTINCT salary) AS 平均,COUNT( DISTINCT salary) AS 去重?cái)?shù)量,COUNT(salary) AS 不去重?cái)?shù)量
FROM employees;

#相當(dāng)于統(tǒng)計(jì)行數(shù)方式一
SELECT COUNT(*) 
FROM employees;

#相當(dāng)于統(tǒng)計(jì)行數(shù)方式二，其中1可以用其他常量或字段替換
SELECT COUNT(1) 
FROM employees;

#其中[]內(nèi)為可選
SELECT 分組函數(shù),列表（要求出現(xiàn)在 GROUP BY 的后面）
FROM 表
[WHERE 篩選條件]
GROUP BY 分組列表
[ORDER BY 子句]

#查詢每個(gè)工種的最高工資
SELECT MAX(salary) AS 最高工資,job_id
FROM employees
GROUP BY job_id;

#查詢每個(gè)部門中，郵箱包含a的員工的平均工資（分組前的篩選）
SELECT AVG(salary) AS 平均工資,department_id
FROM employees
WHERE email LIKE '%a%'
GROUP BY department_id;

#查詢部門員工數(shù)量大于2的部門的員工數(shù)量（分組后的篩選）
#使用HAVING
SELECT COUNT(*) AS 員工數(shù)量,department_id
FROM employees
GROUP BY department_id
HAVING COUNT(*)>2;

#按照多字段
SELECT COUNT(*) AS 員工數(shù)量,job_id,department_id
FROM employees
GROUP BY job_id,department_id;

#完整結(jié)構(gòu)
SELECT AVG(salary) AS 平均工資,department_id
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING AVG(salary)>9000
ORDER BY AVG(salary) DESC;