SQL 三种基本Join

Join是关系型数据库系统的重要操作之一，SQL常用Join：内联接、外联接和交叉联接等。

这里讨论一下这常用的三种连接。

测试环境：db2 v10.1, linux

表定义：

 --用户
 CREATE TABLE USER
 (
 USERID INTEGER NOT NULL,
 COMPANYID INTEGER,
 TELNO VARCHAR(12)
 );

 --公司
 CREATE TABLE COMPANY
 (
 COMPANYID INTEGER NOT NULL,
 TELNO VARCHAR(12)
 );

数据：

--USER

USERID      COMPANYID   TELNO
----------- ----------- ------------
         11           2 777777
         22           3 123456
         33           4 567890  

--COMPANY

COMPANYID   TELNO
----------- ------------
          2 888888   

1. inner join

[db2inst1@win ~]$ db2 "select * from user inner join company on user.companyid=company.companyid"

USERID      COMPANYID   TELNO        COMPANYID   TELNO
----------- ----------- ------------ ----------- ------------
         11           2 777777                 2 888888      

  1 record(s) selected.

注意：内联接（Inner join）满足交换律：“A inner join B” 和 “B inner join A” 是相等的。

查看访问计划：

Optimized Statement:
-------------------
SELECT
  Q2.USERID AS "USERID",
  Q2.COMPANYID AS "COMPANYID",
  Q2.TELNO AS "TELNO",
  Q1.COMPANYID AS "COMPANYID",
  Q1.TELNO AS "TELNO"
FROM
  DB2INST1.COMPANY AS Q1,
  DB2INST1.USER AS Q2
WHERE
  (Q2.COMPANYID = Q1.COMPANYID)

Access Plan:
-----------
    Total Cost:         13.5566
    Query Degree:        1

              Rows
             RETURN
             (   1)
              Cost
               I/O
               |
                1
             HSJOIN
             (   2)
             13.5566
                2
         /-----+------\
        3                1
     TBSCAN           TBSCAN
     (   3)           (   4)
     6.77858          6.77776
        1                1
       |                |
        3                1
 TABLE: DB2INST1  TABLE: DB2INST1
      USER            COMPANY
       Q2               Q1

用HSJOIN的方式进行，DB2对此进行了重写。

2. outer join

外部联接保存一个或两个输入表的所有行，即使无法找到匹配联接谓词的行。

[db2inst1@win ~]$ db2 "select * from user left outer join company on user.companyid=company.companyid"

USERID      COMPANYID   TELNO        COMPANYID   TELNO
----------- ----------- ------------ ----------- ------------
         11           2 777777                 2 888888
         33           4 567890                 - -
         22           3 123456                 - -           

  3 record(s) selected.

[db2inst1@win ~]$ db2 "select * from user right outer join company on user.companyid=company.companyid"

USERID      COMPANYID   TELNO        COMPANYID   TELNO
----------- ----------- ------------ ----------- ------------
         11           2 777777                 2 888888      

  1 record(s) selected.

查看访问计划：

Optimized Statement:
-------------------
SELECT
  Q2.USERID AS "USERID",
  Q2.COMPANYID AS "COMPANYID",
  Q2.TELNO AS "TELNO",
  Q1.COMPANYID AS "COMPANYID",
  Q1.TELNO AS "TELNO"
FROM
  DB2INST1.COMPANY AS Q1
  LEFT OUTER JOIN DB2INST1.USER AS Q2
  ON (Q2.COMPANYID = Q1.COMPANYID)

Access Plan:
-----------
    Total Cost:         13.5566
    Query Degree:        1

              Rows
             RETURN
             (   1)
              Cost
               I/O
               |
                1
             HSJOIN<
             (   2)
             13.5566
                2
         /-----+------\
        3                1
     TBSCAN           TBSCAN
     (   3)           (   4)
     6.77858          6.77776
        1                1
       |                |
        3                1
 TABLE: DB2INST1  TABLE: DB2INST1
      USER            COMPANY
       Q2               Q1

这里也是用的HSJOIN。

3. cross join

交叉联接（cross join）执行两个表的笛卡尔积（就是把表A和表B的数据进行一个N*M的组合）。也就是说，它匹配一个表与另一个表中的每一行；我们不能通过使用ON子句在交叉联接指定谓词，虽然我们可以使用WHERE子句来实现相同的结果，这是交叉联接基本上是作为一个内部联接了。

[db2inst1@win ~]$ db2 "select * from user cross join company"

USERID      COMPANYID   TELNO        COMPANYID   TELNO
----------- ----------- ------------ ----------- ------------
         11           2 777777                 2 888888
         22           3 123456                 2 888888
         33           4 567890                 2 888888      

  3 record(s) selected.

查看访问计划：

Optimized Statement:
-------------------
SELECT
  Q2.USERID AS "USERID",
  Q2.COMPANYID AS "COMPANYID",
  Q2.TELNO AS "TELNO",
  Q1.COMPANYID AS "COMPANYID",
  Q1.TELNO AS "TELNO"
FROM
  DB2INST1.COMPANY AS Q1,
  DB2INST1.USER AS Q2

Access Plan:
-----------
    Total Cost:         13.5563
    Query Degree:        1

              Rows
             RETURN
             (   1)
              Cost
               I/O
               |
                3
             NLJOIN
             (   2)
             13.5563
                2
         /-----+------\
        1                3
     TBSCAN           TBSCAN
     (   3)           (   4)
     6.77776          6.77858
        1                1
       |                |
        1                3
 TABLE: DB2INST1  TABLE: DB2INST1
     COMPANY           USER
       Q1               Q2

看到这里使用的是NLJOIN。