首先，准备两张表
mysql> SELECT * FROM tdb_goods;
+----------+---------------------------------------+---------+----------+-------------+---------+------------+
| goods_id | goods_name | cate_id | brand_id | goods_price | is_show | is_saleoff |
+----------+---------------------------------------+---------+----------+-------------+---------+------------+
| 1 | 笔记本电脑 | 5 | 1 | 3000.000 | 1 | 0 |
| 2 | 足球 | 1 | 9 | 3000.000 | 1 | 0 |
| 3 | 洗发水 | 3 | 2 | 50.000 | 1 | 0 |
| 4 | 红米2A增强版 | 5 | 4 | 800.000 | 1 | 0 |
| 5 | 蒙古牛肉干 | 4 | 6 | 145.000 | 1 | 0 |
| 6 | 蒙古奶酪 | 4 | 6 | 74.000 | 1 | 0 |
| 7 | 十八街麻花 | 2 | 5 | 80.000 | 1 | 0 |
| 8 | 联想游戏本 | 5 | 7 | 7000.000 | 1 | 0 |
| 9 | 苹果笔记本 | 5 | 8 | 8000.000 | 1 | 0 |
| 10 | 外星人II | 5 | 3 | 20000.000 | 1 | 0 |
| 11 | 小米4 | 5 | 4 | 2000.000 | 1 | 0 |
| 13 | LaserJet Pro Ｐ1606dn 黑白激光打印机 | 12 | 4 | 1849.000 | 1 | 0 |
| 14 | 小米4s | 9 | 9 | 5000.000 | 1 | 0 |
| 18 | 华为 | 10 | 10 | 5000.000 | 1 | 0 |
+----------+---------------------------------------+---------+----------+-------------+---------+------------+
14 rows in set (0.00 sec)
mysql> SELECT * FROM tdb_goods_cates;
+---------+------------+
| cate_id | cate_name |
+---------+------------+
| 1 | 体育用品 |
| 2 | 天津特产 |
| 3 | 日用品 |
| 4 | 食品 |
| 5 | 高科技产品 |
| 6 | 路由器 |
| 7 | 交换机 |
| 8 | 网卡 |
+---------+------------+
8 rows in set (0.01 sec)
接下来是简单的操作
第一类子查询：由比较运算符引发
算平均值
SELECT AVG(goods_price) FROM tdb_goods
对取得的结果四舍五入，保留小数点后两位有效 数字
SELECT ROUND(AVG(goods_price),2) FROM tdb_goods;
那么哪些商品的价格大于这个数字？
SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price>= (SELECT ROUND(AVG(goods_price),2) FROM tdb_goods);
SOME，ANY 大于最小值，小于最大值，ALL相反
SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price>= ALL[SOME][ANY](SELECT ROUND(AVG(goods_price),2) FROM tdb_goods);
第二类子查询：使用[NOT]IN的子查询
1 SELECT goods_id ,goods_name ,goods_price FROM tdb_goods WHERE goods_price =ALL(SELECT goods_price FROM tdb_goods WHERE cate_id=5);
2 SELECT goods_id ,goods_name ,goods_price FROM tdb_goods WHERE goods_price !=ALL(SELECT goods_price FROM tdb_goods WHERE cate_id=5);
