1.避免查询重复数据
select DISTINCT field1 from table_name
2.格式化显示查询结果(field2*12表示field2列数据乘12,yearsalary代表前面表达式结果都属于这个字段)
select CONCAT(field1,'your format:',field2*12) yearsalary from table_name
3.模糊查询
查询field1中以字母A开头的数据记录，%匹配多个字符串，_匹配单个字符串
select field1 from table_name where field1 LIKE 'A%';
查询不是以A开头的:
select field1 from table_name where field1 NOT LIKE 'A%';
4.排序：
升序:select * from table_name order by id ASC
降序:select * from table_name order by id DESC
多字段排序:select * from table_name order by id DESC,value ASC
5.限制数据记录查询数量
select * from table_name limit 5;//显示5条数据
select * from table_name limit 5,15; //显示从第6条记录起的15行数据
6.统计函数
select function(field) from table_name
count():统计记录条数
AVG():统计字段平均值
SUM():统计字段总和
MAX():查询字段最大值
MIN():查询字段最小值
7.分组数据--GROUP BY，HAVING
1).创建分组GROUP BY
先列出所有的vend_id，以便作对比
mysql> SELECT vend_id FROM products;
+---------+
| vend_id |
+---------+
| 1001 |
| 1001 |
| 1001 |
| 1002 |
| 1002 |
| 1003 |
| 1003 |
| 1003 |
| 1003 |
| 1003 |
| 1003 |
| 1003 |
| 1005 |
| 1005 |
+---------+
14 rows in set (0.00 sec)
用GROUP BY进行分组
mysql> SELECT vend_id,COUNT(*) AS num_prods
-> FROM products #先分组，再分别计算COUNT（*）
-> GROUP BY vend_id;
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
| 1001 | 3 |
| 1002 | 2 |
| 1003 | 7 |
| 1005 | 2 |
+---------+-----------+
4 rows in set (0.00 sec)
TIPS：
*如果列中有多行NULL值，它们将分为一组
*GROUP BY子句必须出现在WHERE子句之后，ORDER BY子句之前
2).过滤分组HAVING
*HAVING 与 WHERE 的区别：
WHERE过滤行，而HAVING过滤分组
eg:
mysql> SELECT vend_id,COUNT(*) AS num_prods
-> FROM products
-> GROUP BY vend_id
-> HAVING COUNT(*)>2; #从结果中过滤不符合COUNT(*)>2的组
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
| 1001 | 3 |
| 1003 | 7 |
+---------+-----------+
2 rows in set (0.00 sec)
* WHERE在数据分组前进行过滤，HAVING在数据分组后进行过滤，所以，WHERE排除的行不包括在分组中
eg:
先列出原始数据作对比
mysql> SELECT vend_id,prod_price FROM products
-> ORDER BY prod_price;
+---------+------------+
| vend_id | prod_price |
+---------+------------+
| 1003 | 2.50 |
| 1003 | 2.50 |
| 1002 | 3.42 |
| 1003 | 4.49 |
| 1001 | 5.99 |
| 1002 | 8.99 |
| 1001 | 9.99 |
| 1003 | 10.00 |
| 1003 | 10.00 |
| 1003 | 13.00 |
| 1001 | 14.99 |
| 1005 | 35.00 |
| 1003 | 50.00 |
| 1005 | 55.00 |
+---------+------------+
14 rows in set (0.00 sec)
mysql> SELECT vend_id,COUNT(*) AS num_prods
-> FROM products
-> WHERE prod_price >14 #WHERE过滤后只剩下上表中最后3条记录，
-> GROUP BY vend_id #HAVING再过滤分组后vend_id为不符合COUNT(*) >=2组
-> HAVING COUNT(*) >=2;
+---------+-----------+
| vend_id | num_prods |
+---------+-----------+
| 1005 | 2 |
+---------+-----------+
1 row in set (0.00 sec)
3). 分组和排序
GROUP BY和ORDER BY的区别
*ORDER BY指定的条件可以是任意列
*GROUP BY指定的条件只可能使用选择列或列表达式
TIPS：
一般在使用GROUP BY子句时，也应该给出ORDER BY子句
Examples:
先列出原始数据：
mysql> SELECT order_num,quantity,item_price FROM orderitems;
+-----------+----------+------------+
| order_num | quantity | item_price |
+-----------+----------+------------+
| 20005 | 10 | 5.99 |
| 20005 | 3 | 9.99 |
| 20005 | 5 | 10.00 |
| 20005 | 1 | 10.00 |
| 20006 | 1 | 55.00 |
| 20007 | 100 | 10.00 |
| 20008 | 50 | 2.50 |
| 20009 | 1 | 10.00 |
| 20009 | 1 | 8.99 |
| 20009 | 1 | 4.49 |
| 20009 | 1 | 14.99 |
+-----------+----------+------------+
11 rows in set (0.00 sec)
mysql> SELECT order_num,SUM(quantity*item_price) AS ordertotal
-> FROM orderitems
-> GROUP BY order_num
-> HAVING SUM(quantity*item_price) >= 50;
+-----------+------------+ #未用ORDERBY指定排序，结果可能不是想要的，例如按ordertotal升序
| order_num | ordertotal |
+-----------+------------+
| 20005 | 149.87 |
| 20006 | 55.00 |
| 20007 | 1000.00 |
| 20008 | 125.00 |
+-----------+------------+
4 rows in set (0.00 sec)
mysql> SELECT order_num,SUM(quantity*item_price) AS ordertotal
-> FROM orderitems
-> GROUP BY order_num
-> HAVING SUM(quantity*item_price) >=50
-> ORDER BY ordertotal; # 用ORDERBY指定排序方式
+-----------+------------+
| order_num | ordertotal |
+-----------+------------+
| 20006 | 55.00 |
| 20008 | 125.00 |
| 20005 | 149.87 |
| 20007 | 1000.00 |
+-----------+------------+
4 rows in set (0.01 sec)