一、简单查询：
1.基本语法：
SELECT * FROM tb_name;//查询全部 SELECT field1,field2 FROM tb_name; //投影 SELECT [DISTINCT] * FROM tb_name WHERE qualification; //选择
说明：
FROM子句： 要查询的关系 表、多个表、其它SELECT语句
WHERE子句：布尔关系表达式，主要包含如下这几类表达式：
比较：=、>、>=、<=、< 逻辑关系： AND OR NOT BETWEEN ... AND ... ：在两个值之间 LIKE ‘’ %: 任意长度任意字符 _：任意单个字符 REGEXP, RLIKE :正则表达式，此时索引无效 IN IS NULL IS NOT NULL
2.对查询结果排序：
ORDER BY field_name {ASC|DESC}
实例：
mysql> select ID,post_title,post_date from wp_posts where ID>1290 and post_status='publish' ORDER by ID; //升序,ID是排序的字段 mysql> select ID,post_title,post_date from wp_posts where ID>1290 and post_status='publish' ORDER by ID DESC;//降序
3.字段别名：AS
select col_name AS COL_Aliases … //对字段使用别名 select col_name,… from tb_name AS tb_Aliases … //对表使用别名
4.LIMIT子句：
mysql> select ID,post_title,post_date from wp_posts where ID>1290 and post_status='publish' limit 5;//显示匹配到的5条数据 mysql> select ID,post_title,post_date from wp_posts where ID>1290 and post_status='publish' limit2,3; //(逗号前的数字）表示偏移量
5.聚合：SUM(), MIN(), MAX(), AVG(), COUNT()，括号中为字段名
mysql> select sum(ID) from wp_posts;//计算和 mysql> select min(ID) from wp_posts;//查早最小的 mysql> select max(ID) from wp_posts; //查找最大的 mysql> select avg(ID) from wp_posts; //平均值 mysql> select count(ID) from wp_posts;//计数
6.分组：GROUP BY，一般配合聚合运算使用
mysql> select count(post_status) AS 各状态数量,post_status AS 状态名称 from wp_posts group bypost_status; +-----------------+--------------+ | 各状态数量 | 状态名称 | +-----------------+--------------+ | 1 | auto-draft | | 9 | draft | | 251 | inherit | | 238 | publish | | 2 | trash | +-----------------+--------------+ 5 rows in set (0.01 sec)
注意：可以使用HAVING 将GROUP BY的结果再次过滤，用法同where
二、多表查询
连接：
交叉连接：笛卡尔乘积
自然连接：将两张表某字段中相等连接起来，如下
mysql> SELECT students.Name,students.Age,courses.Cname,students.Gender FROM students,courses WHERE students.CID1 = courses.CID; +--------------+------+------------------+--------+ | Name | Age | Cname | Gender | +--------------+------+------------------+--------+ | GuoJing | 19 | TaiJiquan | M | | YangGuo | 17 | TaiJiquan | M | | DingDian | 25 | Qishangquan | M | | HuFei | 31 | Wanliduxing | M | | HuangRong | 16 | Qianzhuwandushou | F | | YueLingshang | 18 | Wanliduxing | F | | ZhangWuji | 20 | Hamagong | M | | Xuzhu | 26 | TaiJiquan | M | +--------------+------+------------------+--------+ 8 rows in set (0.00 sec)
外连接：
左外连接：left_tb LEFT JOIN right_tb ON ... ：以左表为标准
mysql> SELECT s.Name,c.Cname FROM students AS s LEFT JOIN courses AS c ON s.CID1=c.CID; +--------------+--------------------+ | Name | Cname | +--------------+--------------------+ | GuoJing | TaiJiquan | | YangGuo | TaiJiquan | | DingDian | Qishangquan | | HuFei | Wanliduxing | | HuangRong | Qianzhuwandushou | | YueLingshang | Wanliduxing | | ZhangWuji | Hamagong | | Xuzhu | TaiJiquan | | LingHuchong | NULL | | YiLin | NULL | +--------------+--------------------+ 10 rows in set (0.00 sec)
右外连接 : left_tb RIGHT JOIN right_tb ON ... ：以右表为标准
mysql> SELECT s.Name,c.Cname FROM students AS s RIGHT JOIN courses AS c ON s.CID1=c.CID; +--------------+--------------------+ | Name | Cname | +--------------+--------------------+ | GuoJing | TaiJiquan | | YangGuo | TaiJiquan | | DingDian | Qishangquan | | HuFei | Wanliduxing | | HuangRong | Qianzhuwandushou | | YueLingshang | Wanliduxing | | ZhangWuji | Hamagong | | Xuzhu | TaiJiquan | | NULL | Yiyangzhi | | NULL | Jinshejianfa | | NULL | Qiankundanuoyi | | NULL | Pixiejianfa | | NULL | Jiuyinbaiguzhua | +--------------+--------------------+ 13 rows in set (0.01 sec)
自连接：本表中不同字段间进行连接
mysql> SELECT c.Name AS student,s.Name AS teacher FROM students AS c,students AS s WHERE c.TID=s.SID; +-----------+-------------+ | student | teacher | +-----------+-------------+ | GuoJing | DingDian | | YangGuo | GuoJing | | DingDian | ZhangWuji | | HuFei | HuangRong | | HuangRong | LingHuchong | +-----------+-------------+ 5 rows in set (0.02 sec) 注意：使用了别名
三、子查询：一个查询中嵌套另外一个查询
如下：在students表中查询年龄大于平均年龄的学生
mysql> SELECT Name,Age FROM students WHERE Age > (SELECT AVG(Age) FROM students); +-------------+------+ | Name | Age | +-------------+------+ | DingDian | 25 | | HuFei | 31 | | Xuzhu | 26 | | LingHuchong | 22 | +-------------+------+ 4 rows in set (0.08 sec)
子查询注意事项：
比较操作中使用子查询：子查询只能返回单个值；
IN(): 使用子查询；
在FROM中使用子查询；
联合查询：UNION,将两个查询的结果合并
mysql> (SELECT Name,Age FROM students) UNION (SELECT Tname,Age FROM tutors); +--------------+------+ | Name | Age | +--------------+------+ | GuoJing | 19 | | YangGuo | 17 | | DingDian | 25 | | HuFei | 31 | | HuangRong | 16 | | YueLingshang | 18 | | ZhangWuji | 20 | | HuYidao | 42 | | NingZhongze | 49 | +--------------+------+ 19 rows in set (0.00 sec)
