SQL Cookbook阅读记录

SQl CookBook记录

检索记录

• 检索行和列
  • 部分行
  • 条件行
  • SELECT中使用条件

    case when ...

        chen ...

        else ...

    end

     

    • case
  • 返回限制的行数

    DB2-----------------------select * from emp fetch first 5 rows only

    MySQL PostgreSQL ---------select * from emp limit 5

    Oracle -------------------select * from emp where rownum <=5(ROWNUM是在获取每行之后才赋予的)

    SQL Server----------------select top 5 * from emp

     

• 随机返回n条记录

  DB2 --------------------- select * from emp order by rand() fetch first 5 rows only

  MySQL-------------------- select * from emp order by rand() limit 5

  PostgreSQL--------------- select * from emp order by random() limit 5

  Oracle------------------- select * from (select * from emp order by dbms_random.value()) where rownum <= 5

  SQL Server--------------- select top 5 * from emp order by newid()

   

  • order by
• 按模式搜索

  select * from emp where deptno in (10,20)

  select * from emp where ename like '%I%' or job like '%ER'

   

  • in
  • like
• 检索列
  • 部分列
  • 为列取名称
  • WHERE中引用取别名的列

    将查询作为内联视图

    select * from

        (select sal as salary, comm as commission from emp) x

    where salary < 5000

     

    • 内联视图
  • 连接列值

    DB2,Oracle, PostgreSQL --- ||

    MySQL -------------------- concat()

    SQL server --------------- +

     

• 空值
  • 查找空值

    select * from emp where comm is null

     

    • IS NULL
  • 转换空值

    select coalesce(comm,0) from emp

    select case

        when comm is null then 0

        else comm

        end

    from emp

     

    • COALESCE()
    • CASE