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_tbLEFT JOINright_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)