inner left right join 小例子!让你一目了然记住三种连接区别

---首先创建两个表 左表和右表

SQL> CREATE TABLE L AS

  2  SELECT 'left_1' AS str,'1' AS v FROM dual UNION ALL

  3  SELECT 'left_2','2' AS v FROM dual UNION ALL

  4  SELECT 'left_3','3' AS v FROM dual UNION ALL

  5  SELECT 'left_4','4' AS v FROM dual;

表已创建。       

SQL> CREATE TABLE R AS

  2  SELECT 'right_3' AS str,'3' AS v,1 AS status FROM dual UNION ALL

  3  SELECT 'right_4' AS str,'4' AS v,0 AS status FROM dual UNION ALL

  4  SELECT 'right_5' AS str,'5' AS v,0 AS status FROM dual UNION ALL

  5  SELECT 'right_6' AS str,'6' AS v,0 AS status FROM dual;

表已创建。

--查询下L R表数据

SQL> select * from l;

STR    V

------ -

left_1 1

left_2 2

left_3 3

left_4 4

SQL> select * from r;

STR     V     STATUS

------- - ----------

right_3 3          1

right_4 4          0

right_5 5          0

right_6 6          0

---分别显示下 inner join ,left join, right join查询处的数据

SQL> SELECT L.str AS LEFT_str,R.str AS RIGHT_str FROM L INNER JOIN R ON L.v = R.v ORDER BY 1,2;

LEFT_S RIGHT_S

------ -------

left_3 right_3

left_4 right_4

SQL> SELECT L.str AS LEFT_str,R.str AS RIGHT_str FROM L left JOIN R ON L.v = R.v ORDER BY 1,2;

LEFT_S RIGHT_S

------ -------

left_1

left_2

left_3 right_3

left_4 right_4

 

SQL> SELECT L.str AS LEFT_str,R.str AS RIGHT_str FROM L right JOIN R ON L.v = R.v ORDER BY 1,2;

LEFT_S RIGHT_S

------ -------

left_3 right_3

left_4 right_4

       right_5

       right_6

左右连接还有    L.v = R.v(+)      L.v(+)  = R.v  这样写法！ （+）意识是没有数据的用空值填充

egg,关于一个表添加条件是，条件应该加在哪

----对于其中的R表，我们需要只显示其中status = 1的部分    

--true --

SELECT L.str AS LEFT_str,R.str AS RIGHT_str FROM L LEFT JOIN （select * from R where status=1）R ON L.v = R.v ORDER BY 1,2;

 

--error--
SELECT l.str AS left_str, r.str AS right_str  FROM l, r WHERE l.v = r.v(+) AND status = 1 ORDER BY 1, 2;