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
select * from goods where shop_price >= 100 and shop_price <p>取出价格=500<br><br></p><pre class="brush:sql;toolbar:false">select * from goods where 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;
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