How to use the MySQL select statement
select statements can be separated by carriage returns
$sql="select * from article where id=1"和 $sql="select * from article where id=1"都可以得到正确的结果，但有时分开写或许能 更明了一点，特别是当sql语句比较长时。
Batch query data
可以用in 来实现$sql="select * from article where id ;in(1,3,5)"
Use concat to connect the query results
$sql="select concat(id,"-",con) as res from article where id=1"返回 "1-article content"
Use locate
用法： select locate("hello","hello baby");返回1
不存在返回 0Use group by
以前一直没怎么搞明group by 和 order by，其实也满简单的，group by 是把相同的结 果编为一组 exam：$sql="select city ,count(*) from customer group by city"; 这句话的意 思就是从customer表里列出所有不重复的城市，及其数量（有点类似distinct） group by 经常与AVG(),MIN(),MAX(),SUM(),COUNT()一起使用
Use having
having 允许有条件地聚合数据为组$sql="select city,count(*),min(birth_day) from customer group by city having c ount(*)>10"; 这句话是先按city归组，然后找出city地数量大于10的城 市 btw：使用group by + having 速度有点慢 同 时having子句包含的表达式必须在之前出现过
Combining clauses
where、 group by、having、order by（如果这四个都要使用的话，一般按这 个顺序排列）
Use distinct
distinct是去掉重复值用的$sql="select distinct city from customer order by id desc"; 这句话的意思就是从customer表中查 询所有的不重复的city
Use limit
如果要显示某条记录之后的所 有记录$sql="select * from article limit 100,- 1";
Multiple table query
$sql="select user_name from user u,membe r m where u.id=m.id and m.reg_date> ;=2006-12-28 order by u.id desc"
Note: If both user and member have user_name fields at the same time, a mysql error will occur (because mysql does not know which table you want to query for user_name ), you must specify which table it is from.
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