mysql 查

获取表有多少行
技巧：

select count(*) from table_name;

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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;

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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;

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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);

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算出比市场价优惠的数值

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

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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;

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（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)

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作用相同

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

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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;

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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%';

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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%';

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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;

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