獲取表有多少行
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);
取出價格>=100 且<=500
select * from goods where shop_price >= 100 and shop_price <= 500;
select * from goods where shop_price between 100 and 500;
取出價格<=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是散點的集合，between and是區間
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);
算出比市場價優惠的數值
select goods_id,(market_price-shop_price) as chajia ,goods_name from goods ;
查找本地價格比市場價便宜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作用過之後的產生的）
疑點注意：where是對真實表中的數據發揮作用，having可以對where結果進行過濾
select goods_id,(market_price-shop_price) as chajia ,goods_name from goods where chajia > 200;(錯誤的)
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]改爲20 [30,39]改爲30
update mian set num = floor(num/10)*10 where num between 20 and 39;
like模糊查詢
截取諾基亞後面的內容
select goods_id ,goods_name,substring(goods_name,4) from goods where goods_name like '諾基亞%';
查找有諾基亞開頭的更換爲htc（沒有更改真實表內容）
select goods_id ,goods_name,concat('htc',substring(goods_name,4)) from goods where goods_name like '諾基亞%';
把諾基亞更換爲htc（更改真實表內容）
update goods
set goods_name = concat('htc',substring(goods_name,4))
where goods_name like '諾基亞%' and cat_id=4;