六、MySQL查询数据
6.1、基本查询语句
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MySQL从数据表中查询数据的基本语句为SELECT语句，其基本格式为：
SELECT
{* | <字段列表>}
[
FROM <表1>,<表2>...
[WHERE 表达式
[GROUP BY ]
[HAVING [{ }...]]
[ORDER BY ]
[LIMIT [,] ]
]
]
SELECT [字段1,字段2,...,字段n]
FROM [表或视图]
WHERE [查询条件] 各条子句的含义：
{* | <字段列表>}：包含星号通配符选择字段列表，表示查询的字段，其字段列至少包含一个字段名称。 FROM <表1>,<表2>...：表1，表2表示查询数据的来源 WHERE 表达式：可选项，限定查询必须满足的查询条件 GROUP BY <字段>：告诉MySQL如何显示查询出来的数据，并按照指定字段分组 ORDER BY <字段>：告诉MySQL按什么顺序显示查询的数据，可进行的排序ASC,DESC LIMIT [ |
mysql> CREATE TABLE fruits
-> (
-> f_id char(10) NOT NULL,
-> s_id INT NOT NULL,
-> f_name char(255) NOT NULL,
-> f_price decimal(8,2) NOT NULL,
-> PRIMARY KEY(f_id)
-> );
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO fruits (f_id, s_id, f_name, f_price)
-> VALUES('a1', 101,'apple',5.2),
-> ('b1',101,'blackberry', 10.2),
-> ('bs1',102,'orange', 11.2),
-> ('bs2',105,'melon',8.2),
-> ('t1',102,'banana', 10.3),
-> ('t2',102,'grape', 5.3),
-> ('o2',103,'coconut', 9.2),
-> ('c0',101,'cherry', 3.2),
-> ('a2',103, 'apricot',2.2),
-> ('l2',104,'lemon', 6.4),
-> ('b2',104,'berry', 7.6),
-> ('m1',106,'mango', 15.6),
-> ('m2',105,'xbabay', 2.6),
-> ('t4',107,'xbababa', 3.6),
-> ('m3',105,'xxtt', 11.6),
-> ('b5',107,'xxxx', 3.6);
Query OK, 16 rows affected (0.09 sec)
Records: 16 Duplicates: 0 Warnings: 06.2、单表查询
查询所有字段
语法： SELECT * FROM 表名
mysql> SELECT * FROM fruits; +------+------+------------+---------+ | f_id | s_id | f_name | f_price | +------+------+------------+---------+ | a1 | 101 | apple | 5.20 | | a2 | 103 | apricot | 2.20 | | b1 | 101 | blackberry | 10.20 | | b2 | 104 | berry | 7.60 | | b5 | 107 | xxxx | 3.60 | | bs1 | 102 | orange | 11.20 | | bs2 | 105 | melon | 8.20 | | c0 | 101 | cherry | 3.20 | | l2 | 104 | lemon | 6.40 | | m1 | 106 | mango | 15.60 | | m2 | 105 | xbabay | 2.60 | | m3 | 105 | xxtt | 11.60 | | o2 | 103 | coconut | 9.20 | | t1 | 102 | banana | 10.30 | | t2 | 102 | grape | 5.30 | | t4 | 107 | xbababa | 3.60 | +------+------+------------+---------+ 16 rows in set (0.00 sec)
查询指定字段
语法： SELECT 字段名1,字段名2,... FROM 表名
mysql> SELECT f_name FROM fruits; +------------+ | f_name | +------------+ | apple | | apricot | | blackberry | | berry | | xxxx | | orange | | melon | | cherry | | lemon | | mango | | xbabay | | xxtt | | coconut | | banana | | grape | | xbababa | +------------+ 16 rows in set (0.00 sec) mysql> SELECT f_name,f_price FROM fruits; +------------+---------+ | f_name | f_price | +------------+---------+ | apple | 5.20 | | apricot | 2.20 | | blackberry | 10.20 | | berry | 7.60 | | xxxx | 3.60 | | orange | 11.20 | | melon | 8.20 | | cherry | 3.20 | | lemon | 6.40 | | mango | 15.60 | | xbabay | 2.60 | | xxtt | 11.60 | | coconut | 9.20 | | banana | 10.30 | | grape | 5.30 | | xbababa | 3.60 | +------------+---------+ 16 rows in set (0.00 sec)
查询指定记录
语法： SELECT 字段名1,字段名2,... FROM 表名 WHERE 查询条件
WHERE条件判断符
| 操作符 | 说明 |
| = | 相等 |
| <>,!= | 不等于 |
| < | 小于 |
| <= | 小于等于 |
| > | 大于 |
| >= | 大于等于 |
| BETWEEN | 位于两值之间 |
mysql> SELECT f_name, f_price -> FROM fruits -> WHERE f_price = 10.2; +------------+---------+ | f_name | f_price | +------------+---------+ | blackberry | 10.20 | +------------+---------+ 1 row in set (0.00 sec) mysql> SELECT f_name, f_price -> FROM fruits -> WHERE f_name = 'apple'; +--------+---------+ | f_name | f_price | +--------+---------+ | apple | 5.20 | +--------+---------+ 1 row in set (0.00 sec) mysql> SELECT f_name, f_price -> FROM fruits -> WHERE f_price < 10; +---------+---------+ | f_name | f_price | +---------+---------+ | apple | 5.20 | | apricot | 2.20 | | berry | 7.60 | | xxxx | 3.60 | | melon | 8.20 | | cherry | 3.20 | | lemon | 6.40 | | xbabay | 2.60 | | coconut | 9.20 | | grape | 5.30 | | xbababa | 3.60 | +---------+---------+ 11 rows in set (0.00 sec)
带IN关键字的查询
IN操作符用来查询满足指定范围内的条件记录，使用IN操作符，将所有检索条件用括号括起来，检索条件之间用逗号分隔开，只要满足条件范围内的一个值即可。
mysql> SELECT s_id,f_name, f_price -> FROM fruits -> WHERE s_id IN (101,102) -> ORDER BY f_name; +------+------------+---------+ | s_id | f_name | f_price | +------+------------+---------+ | 101 | apple | 5.20 | | 102 | banana | 10.30 | | 101 | blackberry | 10.20 | | 101 | cherry | 3.20 | | 102 | grape | 5.30 | | 102 | orange | 11.20 | +------+------------+---------+ 6 rows in set (0.00 sec) mysql> SELECT s_id,f_name, f_price -> FROM fruits -> WHERE s_id NOT IN (101,102) -> ORDER BY f_name; +------+---------+---------+ | s_id | f_name | f_price | +------+---------+---------+ | 103 | apricot | 2.20 | | 104 | berry | 7.60 | | 103 | coconut | 9.20 | | 104 | lemon | 6.40 | | 106 | mango | 15.60 | | 105 | melon | 8.20 | | 107 | xbababa | 3.60 | | 105 | xbabay | 2.60 | | 105 | xxtt | 11.60 | | 107 | xxxx | 3.60 | +------+---------+---------+ 10 rows in set (0.00 sec)
带BETWEEN AND的范围查询
BETWEEN AND用来查询某个范围内的值，该操作符需要两个参数，即范围的开始和结束，如果字段满足指定的范围查询条件，则这些记录被返回。
mysql> SELECT f_name, f_price -> FROM fruits -> WHERE f_price BETWEEN 2.00 AND 10.20; +------------+---------+ | f_name | f_price | +------------+---------+ | apple | 5.20 | | apricot | 2.20 | | blackberry | 10.20 | | berry | 7.60 | | xxxx | 3.60 | | melon | 8.20 | | cherry | 3.20 | | lemon | 6.40 | | xbabay | 2.60 | | coconut | 9.20 | | grape | 5.30 | | xbababa | 3.60 | +------------+---------+ 12 rows in set (0.00 sec) mysql> SELECT f_name, f_price -> FROM fruits -> WHERE f_price NOT BETWEEN 2.00 AND 10.20; +--------+---------+ | f_name | f_price | +--------+---------+ | orange | 11.20 | | mango | 15.60 | | xxtt | 11.60 | | banana | 10.30 | +--------+---------+ 4 rows in set (0.00 sec)
带LIKE的字符匹配查询
%通配符，匹配任意长度任意字符
mysql> SELECT f_id, f_name -> FROM fruits -> WHERE f_name LIKE 'b%'; +------+------------+ | f_id | f_name | +------+------------+ | b1 | blackberry | | b2 | berry | | t1 | banana | +------+------------+ 3 rows in set (0.00 sec) mysql> SELECT f_id, f_name -> FROM fruits -> WHERE f_name LIKE '%g%'; +------+--------+ | f_id | f_name | +------+--------+ | bs1 | orange | | m1 | mango | | t2 | grape | +------+--------+ 3 rows in set (0.00 sec)
_通配符：匹配单个字符
mysql> SELECT f_id, f_name FROM fruits WHERE f_name LIKE '____e'; +------+--------+ | f_id | f_name | +------+--------+ | a1 | apple | | t2 | grape | +------+--------+ 2 rows in set (0.00 sec)
查询空值
数据表创建的时候，可以指定某列中可以包含空值（NULL），空值不同于0，也不同于空字符串。空值一般表示数据未知、不适用或将在以后添加数据。使用IS NULL子句可以查询某字段内容为空的记录。
mysql> CREATE TABLE customers -> ( -> c_id int NOT NULL AUTO_INCREMENT, -> c_name char(50) NOT NULL, -> c_address char(50) NULL, -> c_city char(50) NULL, -> c_zip char(10) NULL, -> c_contact char(50) NULL, -> c_email char(255) NULL, -> PRIMARY KEY (c_id) -> ); Query OK, 0 rows affected (0.02 sec) mysql> INSERT INTO customers(c_id, c_name, c_address, c_city, -> c_zip, c_contact, c_email) -> VALUES(10001, 'RedHook', '200 Street ', 'Tianjin', -> '300000', 'LiMing', 'LMing@163.com'), -> (10002, 'Stars', '333 Fromage Lane', -> 'Dalian', '116000', 'Zhangbo','Jerry@hotmail.com'), -> (10003, 'Netbhood', '1 Sunny Place', 'Qingdao', '266000', -> 'LuoCong', NULL), -> (10004, 'JOTO', '829 Riverside Drive', 'Haikou', -> '570000', 'YangShan', 'sam@hotmail.com'); Query OK, 4 rows affected (0.02 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql> SELECT c_id, c_name,c_email FROM customers WHERE c_email IS NULL; +-------+----------+---------+ | c_id | c_name | c_email | +-------+----------+---------+ | 10003 | Netbhood | NULL | +-------+----------+---------+ 1 row in set (0.00 sec) mysql> SELECT c_id, c_name,c_email FROM customers WHERE c_email IS NOT NULL; +-------+---------+-------------------+ | c_id | c_name | c_email | +-------+---------+-------------------+ | 10001 | RedHook | LMing@163.com | | 10002 | Stars | Jerry@hotmail.com | | 10004 | JOTO | sam@hotmail.com | +-------+---------+-------------------+ 3 rows in set (0.00 sec)
带AND的多条件查询
mysql> SELECT f_id, f_price, f_name FROM fruits WHERE s_id = '101' AND f_price >=5;
+------+---------+------------+
| f_id | f_price | f_name |
+------+---------+------------+
| a1 | 5.20 | apple |
| b1 | 10.20 | blackberry |
+------+---------+------------+
2 rows in set (0.00 sec)
mysql> SELECT f_id, f_price, f_name FROM fruits
-> WHERE s_id IN('101', '102') AND f_price >= 5 AND f_name = 'apple';
+------+---------+--------+
| f_id | f_price | f_name |
+------+---------+--------+
| a1 | 5.20 | apple |
+------+---------+--------+
1 row in set (0.00 sec)带OR的多条件查询
mysql> SELECT s_id,f_name, f_price -> FROM fruits -> WHERE s_id = 101 OR s_id = 102; +------+------------+---------+ | s_id | f_name | f_price | +------+------------+---------+ | 101 | apple | 5.20 | | 101 | blackberry | 10.20 | | 102 | orange | 11.20 | | 101 | cherry | 3.20 | | 102 | banana | 10.30 | | 102 | grape | 5.30 | +------+------------+---------+ 6 rows in set (0.00 sec) mysql> SELECT s_id,f_name, f_price -> FROM fruits -> WHERE s_id IN(101,102); +------+------------+---------+ | s_id | f_name | f_price | +------+------------+---------+ | 101 | apple | 5.20 | | 101 | blackberry | 10.20 | | 102 | orange | 11.20 | | 101 | cherry | 3.20 | | 102 | banana | 10.30 | | 102 | grape | 5.30 | +------+------------+---------+ 6 rows in set (0.00 sec)
OR操作符和IN操作符使用后结果一样，可以实现相同的功能。但是使用IN操作符使检索语句更加简明，并且IN执行的速度更快于OR。
查询结果不重复
MySQL中使用DISTINCT关键字消除重复的记录，其语法为：
SELECT DISTINCT 字段名 FROM 表名
mysql> SELECT s_id FROM fruits; +------+ | s_id | +------+ | 101 | | 103 | | 101 | | 104 | | 107 | | 102 | | 105 | | 101 | | 104 | | 106 | | 105 | | 105 | | 103 | | 102 | | 102 | | 107 | +------+ 16 rows in set (0.00 sec) mysql> SELECT DISTINCT s_id FROM fruits; +------+ | s_id | +------+ | 101 | | 103 | | 104 | | 107 | | 102 | | 105 | | 106 | +------+ 7 rows in set (0.00 sec)
对查询的结果排序
单列排序
mysql> SELECT f_name FROM fruits; +------------+ | f_name | +------------+ | apple | | apricot | | blackberry | | berry | | xxxx | | orange | | melon | | cherry | | lemon | | mango | | xbabay | | xxtt | | coconut | | banana | | grape | | xbababa | +------------+ 16 rows in set (0.00 sec) mysql> SELECT f_name FROM fruits ORDER BY f_name; +------------+ | f_name | +------------+ | apple | | apricot | | banana | | berry | | blackberry | | cherry | | coconut | | grape | | lemon | | mango | | melon | | orange | | xbababa | | xbabay | | xxtt | | xxxx | +------------+ 16 rows in set (0.00 sec)
多列排序
mysql> SELECT f_name, f_price FROM fruits ORDER BY f_name, f_price; +------------+---------+ | f_name | f_price | +------------+---------+ | apple | 5.20 | | apricot | 2.20 | | banana | 10.30 | | berry | 7.60 | | blackberry | 10.20 | | cherry | 3.20 | | coconut | 9.20 | | grape | 5.30 | | lemon | 6.40 | | mango | 15.60 | | melon | 8.20 | | orange | 11.20 | | xbababa | 3.60 | | xbabay | 2.60 | | xxtt | 11.60 | | xxxx | 3.60 | +------------+---------+ 16 rows in set (0.00 sec)
指定方向排序
mysql> SELECT f_name, f_price FROM fruits ORDER BY f_price DESC; +------------+---------+ | f_name | f_price | +------------+---------+ | mango | 15.60 | | xxtt | 11.60 | | orange | 11.20 | | banana | 10.30 | | blackberry | 10.20 | | coconut | 9.20 | | melon | 8.20 | | berry | 7.60 | | lemon | 6.40 | | grape | 5.30 | | apple | 5.20 | | xxxx | 3.60 | | xbababa | 3.60 | | cherry | 3.20 | | xbabay | 2.60 | | apricot | 2.20 | +------------+---------+ 16 rows in set (0.00 sec) mysql> SELECT f_price, f_name FROM fruits ORDER BY f_price DESC, f_name; +---------+------------+ | f_price | f_name | +---------+------------+ | 15.60 | mango | | 11.60 | xxtt | | 11.20 | orange | | 10.30 | banana | | 10.20 | blackberry | | 9.20 | coconut | | 8.20 | melon | | 7.60 | berry | | 6.40 | lemon | | 5.30 | grape | | 5.20 | apple | | 3.60 | xbababa | | 3.60 | xxxx | | 3.20 | cherry | | 2.60 | xbabay | | 2.20 | apricot | +---------+------------+ 16 rows in set (0.00 sec)
分组查询
分组查询是对数据按照某个或多个字段进行分组，MySQL中使用GROUP BY 关键字对数据进行分组，基本语法形式为：
[GROUP BY 字段] [HAVING <条件表达式>]
创建分组
GROUP BY关键字通常和集合函数一起使用，如MAX()、MIN()、COUNT()、SUM()、AVG()。
mysql> SELECT s_id, COUNT(*) AS Total FROM fruits GROUP BY s_id; +------+-------+ | s_id | Total | +------+-------+ | 101 | 3 | | 102 | 3 | | 103 | 2 | | 104 | 2 | | 105 | 3 | | 106 | 1 | | 107 | 2 | +------+-------+ 7 rows in set (0.00 sec) mysql> SELECT s_id, GROUP_CONCAT(f_name) AS Names FROM fruits GROUP BY s_id; +------+-------------------------+ | s_id | Names | +------+-------------------------+ | 101 | apple,blackberry,cherry | | 102 | grape,banana,orange | | 103 | apricot,coconut | | 104 | lemon,berry | | 105 | xbabay,xxtt,melon | | 106 | mango | | 107 | xxxx,xbababa | +------+-------------------------+ 7 rows in set (0.00 sec)
使用HAVING过滤分组
mysql> SELECT s_id, GROUP_CONCAT(f_name) AS Names -> FROM fruits -> GROUP BY s_id HAVING COUNT(f_name) > 1; +------+-------------------------+ | s_id | Names | +------+-------------------------+ | 101 | apple,blackberry,cherry | | 102 | grape,banana,orange | | 103 | apricot,coconut | | 104 | lemon,berry | | 105 | xbabay,xxtt,melon | | 107 | xxxx,xbababa | +------+-------------------------+ 6 rows in set (0.00 sec)
在GROUP BY 子句中使用WITH ROLLUP，显示查询出的所有记录总和
mysql> SELECT s_id, COUNT(*) AS Total -> FROM fruits -> GROUP BY s_id WITH ROLLUP; +------+-------+ | s_id | Total | +------+-------+ | 101 | 3 | | 102 | 3 | | 103 | 2 | | 104 | 2 | | 105 | 3 | | 106 | 1 | | 107 | 2 | | NULL | 16 | +------+-------+ 8 rows in set (0.00 sec)
多字段分组
mysql> SELECT * FROM fruits group by s_id,f_name; +------+------+------------+---------+ | f_id | s_id | f_name | f_price | +------+------+------------+---------+ | a1 | 101 | apple | 5.20 | | b1 | 101 | blackberry | 10.20 | | c0 | 101 | cherry | 3.20 | | t1 | 102 | banana | 10.30 | | t2 | 102 | grape | 5.30 | | bs1 | 102 | orange | 11.20 | | a2 | 103 | apricot | 2.20 | | o2 | 103 | coconut | 9.20 | | b2 | 104 | berry | 7.60 | | l2 | 104 | lemon | 6.40 | | bs2 | 105 | melon | 8.20 | | m2 | 105 | xbabay | 2.60 | | m3 | 105 | xxtt | 11.60 | | m1 | 106 | mango | 15.60 | | t4 | 107 | xbababa | 3.60 | | b5 | 107 | xxxx | 3.60 | +------+------+------------+---------+ 16 rows in set (0.00 sec)
GROUP BY 和ORDER BY一起使用
mysql> CREATE TABLE orderitems -> ( -> o_num int NOT NULL, -> o_item int NOT NULL, -> f_id char(10) NOT NULL, -> quantity int NOT NULL, -> item_price decimal(8,2) NOT NULL, -> PRIMARY KEY (o_num,o_item) -> ) ; Query OK, 0 rows affected (0.09 sec) mysql> INSERT INTO orderitems(o_num, o_item, f_id, quantity, item_price) -> VALUES(30001, 1, 'a1', 10, 5.2), -> (30001, 2, 'b2', 3, 7.6), -> (30001, 3, 'bs1', 5, 11.2), -> (30001, 4, 'bs2', 15, 9.2), -> (30002, 1, 'b3', 2, 20.0), -> (30003, 1, 'c0', 100, 10), -> (30004, 1, 'o2', 50, 2.50), -> (30005, 1, 'c0', 5, 10), -> (30005, 2, 'b1', 10, 8.99), -> (30005, 3, 'a2', 10, 2.2), -> (30005, 4, 'm1', 5, 14.99); Query OK, 11 rows affected (0.08 sec) Records: 11 Duplicates: 0 Warnings: 0 mysql> SELECT o_num, SUM(quantity * item_price) AS orderTotal -> FROM orderitems -> GROUP BY o_num -> HAVING SUM(quantity*item_price) >= 100; +-------+------------+ | o_num | orderTotal | +-------+------------+ | 30001 | 268.80 | | 30003 | 1000.00 | | 30004 | 125.00 | | 30005 | 236.85 | +-------+------------+ 4 rows in set (0.00 sec) mysql> SELECT o_num, SUM(quantity * item_price) AS orderTotal -> FROM orderitems -> GROUP BY o_num -> HAVING SUM(quantity*item_price) >= 100 -> ORDER BY orderTotal; +-------+------------+ | o_num | orderTotal | +-------+------------+ | 30004 | 125.00 | | 30005 | 236.85 | | 30001 | 268.80 | | 30003 | 1000.00 | +-------+------------+ 4 rows in set (0.00 sec)
使用LIMIT限制查询结果数量
使用LIMIT关键字，可以限制查询结果的数量，语法格式为：
LIMIT [位置偏移量,] 行数
mysql> SELECT * From fruits LIMIT 4; +------+------+------------+---------+ | f_id | s_id | f_name | f_price | +------+------+------------+---------+ | a1 | 101 | apple | 5.20 | | a2 | 103 | apricot | 2.20 | | b1 | 101 | blackberry | 10.20 | | b2 | 104 | berry | 7.60 | +------+------+------------+---------+ 4 rows in set (0.00 sec) mysql> SELECT * From fruits LIMIT 4, 3; +------+------+--------+---------+ | f_id | s_id | f_name | f_price | +------+------+--------+---------+ | b5 | 107 | xxxx | 3.60 | | bs1 | 102 | orange | 11.20 | | bs2 | 105 | melon | 8.20 | +------+------+--------+---------+ 3 rows in set (0.00 sec)
6.3、使用聚合函数查询
有时不需要返回实际表中的数据，而只是针对数据进行总结。MySQL提供一些查询功能，可以获取数据进行分析和报告。
| 函数 | 作用 |
| AVG() | 返回某列的平均值 |
| COUNT() | 返回某列的行数 |
| MAX() | 返回某列的最大值 |
| MIN() | 返回某列的最小值 |
| SUM() | 返回某列值的和 |
COUNT()函数
mysql> SELECT COUNT(*) AS cust_num -> FROM customers; +----------+ | cust_num | +----------+ | 4 | +----------+ 1 row in set (0.00 sec) mysql> SELECT COUNT(c_email) AS email_num -> FROM customers; +-----------+ | email_num | +-----------+ | 3 | +-----------+ 1 row in set (0.00 sec) mysql> SELECT o_num, COUNT(f_id) -> FROM orderitems -> GROUP BY o_num; +-------+-------------+ | o_num | COUNT(f_id) | +-------+-------------+ | 30001 | 4 | | 30002 | 1 | | 30003 | 1 | | 30004 | 1 | | 30005 | 4 | +-------+-------------+ 5 rows in set (0.00 sec)
SUM()函数
mysql> SELECT SUM(quantity) AS items_total -> FROM orderitems -> WHERE o_num = 30005; +-------------+ | items_total | +-------------+ | 30 | +-------------+ 1 row in set (0.00 sec) mysql> SELECT o_num, SUM(quantity) AS items_total -> FROM orderitems -> GROUP BY o_num; +-------+-------------+ | o_num | items_total | +-------+-------------+ | 30001 | 33 | | 30002 | 2 | | 30003 | 100 | | 30004 | 50 | | 30005 | 30 | +-------+-------------+ 5 rows in set (0.00 sec)
AVG()函数
mysql> SELECT AVG(f_price) AS avg_price -> FROM fruits -> WHERE s_id = 103; +-----------+ | avg_price | +-----------+ | 5.700000 | +-----------+ 1 row in set (0.00 sec) mysql> SELECT s_id,AVG(f_price) AS avg_price -> FROM fruits -> GROUP BY s_id; +------+-----------+ | s_id | avg_price | +------+-----------+ | 101 | 6.200000 | | 102 | 8.933333 | | 103 | 5.700000 | | 104 | 7.000000 | | 105 | 7.466667 | | 106 | 15.600000 | | 107 | 3.600000 | +------+-----------+ 7 rows in set (0.00 sec)
MAX()函数
mysql> SELECT MAX(f_price) AS max_price FROM fruits; +-----------+ | max_price | +-----------+ | 15.60 | +-----------+ 1 row in set (0.00 sec) mysql> SELECT s_id, MAX(f_price) AS max_price -> FROM fruits -> GROUP BY s_id; +------+-----------+ | s_id | max_price | +------+-----------+ | 101 | 10.20 | | 102 | 11.20 | | 103 | 9.20 | | 104 | 7.60 | | 105 | 11.60 | | 106 | 15.60 | | 107 | 3.60 | +------+-----------+ 7 rows in set (0.00 sec) mysql> SELECT MAX(f_name) FROM fruits; +-------------+ | MAX(f_name) | +-------------+ | xxxx | +-------------+ 1 row in set (0.00 sec)
MIN()函数
mysql> SELECT MIN(f_price) AS min_price FROM fruits; +-----------+ | min_price | +-----------+ | 2.20 | +-----------+ 1 row in set (0.00 sec) mysql> SELECT s_id, MIN(f_price) AS min_price -> FROM fruits -> GROUP BY s_id; +------+-----------+ | s_id | min_price | +------+-----------+ | 101 | 3.20 | | 102 | 5.30 | | 103 | 2.20 | | 104 | 6.40 | | 105 | 2.60 | | 106 | 15.60 | | 107 | 3.60 | +------+-----------+ 7 rows in set (0.00 sec)
6.4、连接查询
连接是关系数据库模型的主要特点。连接查询是关系数据库中最主要的查询。主要包括内连接、外连接等。通过连接元算符可以实现多个表查询。在关系数据库管理系统中，表建立时各个数据之间关系不确定，常把一个实体的所有信息存在一个表中。当查询时，通过连接操作查询出存在多个表中的不同实体的信息。
内连接查询
内连接使用比较运算符进行表间某些列数据的比较操作，并列出这些表中与连接条件相匹配的数据行，组合成新记录。
首先创建表与插入数据：
mysql> CREATE TABLE suppliers -> ( -> s_id int NOT NULL AUTO_INCREMENT, -> s_name char(50) NOT NULL, -> s_city char(50) NULL, -> s_zip char(10) NULL, -> s_call CHAR(50) NOT NULL, -> PRIMARY KEY (s_id) -> ) ; Query OK, 0 rows affected (0.03 sec) mysql> INSERT INTO suppliers(s_id, s_name,s_city, s_zip, s_call) -> VALUES(101,'FastFruit Inc.','Tianjin','300000','48075'), -> (102,'LT Supplies','Chongqing','400000','44333'), -> (103,'ACME','Shanghai','200000','90046'), -> (104,'FNK Inc.','Zhongshan','528437','11111'), -> (105,'Good Set','Taiyuang','030000', '22222'), -> (106,'Just Eat Ours','Beijing','010', '45678'), -> (107,'DK Inc.','Zhengzhou','450000', '33332'); Query OK, 7 rows affected (0.01 sec) Records: 7 Duplicates: 0 Warnings: 0
在fruits表和suppliers表之间使用内连接查询
mysql> SELECT suppliers.s_id, s_name,f_name, f_price -> FROM fruits ,suppliers -> WHERE fruits.s_id = suppliers.s_id; +------+----------------+------------+---------+ | s_id | s_name | f_name | f_price | +------+----------------+------------+---------+ | 101 | FastFruit Inc. | apple | 5.20 | | 103 | ACME | apricot | 2.20 | | 101 | FastFruit Inc. | blackberry | 10.20 | | 104 | FNK Inc. | berry | 7.60 | | 107 | DK Inc. | xxxx | 3.60 | | 102 | LT Supplies | orange | 11.20 | | 105 | Good Set | melon | 8.20 | | 101 | FastFruit Inc. | cherry | 3.20 | | 104 | FNK Inc. | lemon | 6.40 | | 106 | Just Eat Ours | mango | 15.60 | | 105 | Good Set | xbabay | 2.60 | | 105 | Good Set | xxtt | 11.60 | | 103 | ACME | coconut | 9.20 | | 102 | LT Supplies | banana | 10.30 | | 102 | LT Supplies | grape | 5.30 | | 107 | DK Inc. | xbababa | 3.60 | +------+----------------+------------+---------+ 16 rows in set (0.00 sec) mysql> SELECT suppliers.s_id, s_name,f_name, f_price -> FROM fruits INNER JOIN suppliers -> ON fruits.s_id = suppliers.s_id; +------+----------------+------------+---------+ | s_id | s_name | f_name | f_price | +------+----------------+------------+---------+ | 101 | FastFruit Inc. | apple | 5.20 | | 103 | ACME | apricot | 2.20 | | 101 | FastFruit Inc. | blackberry | 10.20 | | 104 | FNK Inc. | berry | 7.60 | | 107 | DK Inc. | xxxx | 3.60 | | 102 | LT Supplies | orange | 11.20 | | 105 | Good Set | melon | 8.20 | | 101 | FastFruit Inc. | cherry | 3.20 | | 104 | FNK Inc. | lemon | 6.40 | | 106 | Just Eat Ours | mango | 15.60 | | 105 | Good Set | xbabay | 2.60 | | 105 | Good Set | xxtt | 11.60 | | 103 | ACME | coconut | 9.20 | | 102 | LT Supplies | banana | 10.30 | | 102 | LT Supplies | grape | 5.30 | | 107 | DK Inc. | xbababa | 3.60 | +------+----------------+------------+---------+ 16 rows in set (0.00 sec)
如果在一个连接查询中，设计的两个表是同一张表，这种查询称为字连接查询。例如：查询供应f_id= ‘a1’的水果供应商提供的其他水果种类。
mysql> SELECT f1.f_id, f1.f_name -> FROM fruits AS f1, fruits AS f2 -> WHERE f1.s_id = f2.s_id AND f2.f_id = 'a1'; +------+------------+ | f_id | f_name | +------+------------+ | a1 | apple | | b1 | blackberry | | c0 | cherry | +------+------------+ 3 rows in set (0.00 sec)
外间接查询
外连接查询将查询多个表中相关联的行，其分为左外连接和右外连接
左外连接：返回包括左表中的所有记录和右表中连接字段相等的记录
首先创建表和数据：
mysql> CREATE TABLE orders
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