select查询

SELECT 语句的常用方法：

• 基本方法
• 查询条件(WHERE;=><;BETWEEN AND)
• AND;OR;IN;OR IN;
• 模糊查询（LIKE;’_’;’%’）
• 对查询排序(ORDER BY:ASC;DESC)
• SQL内置函数和计算(COUNT;AVG;SUM;MAX;MIN)
• 子查询与连接查询

-- 子查询
SELECT of_dpt,COUNT(proj_name) AS count_project FROM project GROUP BY of_dpt
HAVING of_dpt IN
(SELECT in_dpt FROM employee WHERE name='Tom');

-- 连接查询
SELECT id,name,people_num
FROM employee,department
WHERE employee.in_dpt = department.dpt_name
ORDER BY id;
-- 等价于（on后的表名可省略）
SELECT id,name,people_num
FROM employee JOIN department
ON employee.in_dpt = department.dpt_name
ORDER BY id;

例：查询员工所在部门的人数及工程数

• table1;table2;table3
  
  
• 查询方法1
  
  
• 查询方法2