zoukankan      html  css  js  c++  java
  • simple =, join,left out join,right outer join, cross join

     a. =
           eg.
           Select A.a B.b from A, B where A.a=B.a and A.c=’herengang’;
         under this condition, it only shows the data that A.a=B.a and A.c=”herengang”.
       Although it there is data which .c is “herengang” ,but if we  can’t find such record which .a value equals to A.a in Table B, then it will be cleared.
     b.  left join
        select A.a, B.a 
        from table  A
                 left join table B
                on  A.a =B.a and A.c=”herengang”
           under this condition, all of data which .c is “herengang” will be retrieved, and then if B.a exists, then show the value of B.a .

    e.g Table A                                                Table B
          columnA columnB columnC              columnA columnB columnC    
               1            3            a                       1         'ok'     23.3
               2            4            b                       3          'sorry'    3.0

    A join B (A inner B) ,result is 
         1       3        a      1     'ok'      23.3
    A left join B, result is 
         1       3       a       1    'ok'       23.3
         2       4       b     null   null    null
    A cross join B
          1       3       a      1    'ok'      23.3
          1       3       a      3   'sorry'   3.0
          2       4       b      1    'ok'      23.3
          2       4       b      3    'sorry'    3.0
  • 相关阅读:
    CSS:CSS 组合选择符
    CSS:CSS 布局
    CSS:CSS Float(浮动)
    CSS:CSS Positioning(定位)
    CSS:CSS Display(显示) 与 Visibility(可见性)
    CSS:CSS 尺寸 (Dimension)
    CSS:CSS 分组 和 嵌套 选择器
    CSS:CSS padding(填充)
    CSS:CSS margin(外边距)
    linux概念之/proc与/sys
  • 原文地址:https://www.cnblogs.com/Winston/p/1040997.html
Copyright © 2011-2022 走看看