获取表有多少行

技巧：select count(*) from table_name;

取出cat_id=4和cat_id=11的列

使用or   select * from goods where cat_id=4 or cat_id=11;

不使用or  select * from goods where cat_id in(4,11); 

取出价格>=100 且<=500

select * from goods where shop_price >= 100 and shop_price <= 500;

select * from goods where shop_price between 100 and 500;

取出价格<=100 且>=500

select * from goods where shop_price <=100 and shop_price >= 500;

select * from goods where shop_price not between 100 and 500;

in是散点的集合，between and是区间 

cat_id不是3也不是11的列

select * from goods where cat_id!=3 and cat_id!=11;

select * from goods where cat_id not in(3,11);

算出比市场价优惠的数值

select goods_id,(market_price-shop_price) as chajia ,goods_name from goods ;

查找本地价格比市场价便宜200以上的

select goods_id,(market_price-shop_price) as chajia ,goods_name from goods where (market_price - shop_price) > 200;

（chajia列是where作用过之后的产生的）

疑点注意：where是对真实表中的数据发挥作用，having可以对where结果进行过滤

select goods_id,(market_price-shop_price) as chajia ,goods_name from goods where chajia > 200;(错误的)

作用相同

select goods_id,(market_price-shop_price) as chajia ,goods_name from goods having chajia>200;

把mian表中的num列中 [20,29]改为20  [30,39]改为30

update mian set num = floor(num/10)*10 where num between 20 and 39;

like模糊查询

截取诺基亚后面的内容

select goods_id ,goods_name,substring(goods_name,4) from goods where goods_name like '诺基亚%';

查找有诺基亚开头的更换为htc（没有更改真实表内容）

select goods_id ,goods_name,concat('htc',substring(goods_name,4)) from goods where goods_name like '诺基亚%';

把诺基亚更换为htc（更改真实表内容）

update goods 

set goods_name = concat('htc',substring(goods_name,4))

where goods_name like '诺基亚%' and cat_id=4;