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更新时间：2024-10-10 GMT+08:00
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GaussDB(DWS)多表连接查询

连接类型介绍

通过SQL完成各种复杂的查询，多表之间的连接是必不可少的。连接分为：内连接和外连接两大类，每大类中还可进行细分。

  • 内连接：标准内连接（INNER JOIN），交叉连接（CROSS JOIN）和自然连接（NATURAL JOIN）。
  • 外连接：左外连接（LEFT OUTER JOIN），右外连接（RIGHT OUTER JOIN）和全外连接（FULL JOIN）。

为了能更好的说明各种连接之间的区别，下面通过具体示例进行详细的阐述。

创建示例表student和math_score，并插入数据，设置enable_fast_query_shipping为off（默认为on）即查询优化器使用分布式框架；参数explain_perf_mode为pretty（默认值为pretty）指定explain的显示格式。

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CREATE TABLE student(
  id INTEGER,
  name varchar(50)
);

CREATE TABLE math_score(
  id INTEGER,
  score INTEGER
);

INSERT INTO student VALUES(1, 'Tom');
INSERT INTO student VALUES(2, 'Lily');
INSERT INTO student VALUES(3, 'Tina');
INSERT INTO student VALUES(4, 'Perry');

INSERT INTO math_score VALUES(1, 80);
INSERT INTO math_score VALUES(2, 75);
INSERT INTO math_score VALUES(4, 95);
INSERT INTO math_score VALUES(6, NULL);

SET enable_fast_query_shipping = off;
SET explain_perf_mode = pretty;

内连接

  • 标准内连接（INNER JOIN）
    语法： 
    1
    		 
    left_table [INNER] JOIN right_table [ ON join_condition | USING ( join_column )]

    说明：表示left_table和right_table中满足join_condition的行拼接在一起作为结果输出，不满足条件的元组不会输出。

    示例1：查询学生的数学成绩。

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    SELECT s.id, s.name, ms.score FROM student s JOIN math_score ms on s.id = ms.id;
 id | name  | score
----+-------+-------
  2 | Lily  |    75
  1 | Tom   |    80
  4 | Perry |    95
(3 rows)


EXPLAIN SELECT s.id, s.name, ms.score FROM student s JOIN math_score ms on s.id = ms.id;
                                       QUERY PLAN
----------------------------------------------------------------------------------------
  id |                operation                | E-rows | E-memory | E-width | E-costs
 ----+-----------------------------------------+--------+----------+---------+---------
   1 | ->  Streaming (type: GATHER)            |      4 |          |      13 | 19.47
   2 |    ->  Hash Join (3,4)                  |      4 | 1MB      |      13 | 11.47
   3 |       ->  Seq Scan on math_score ms     |     30 | 1MB      |       8 | 10.10
   4 |       ->  Hash                          |     12 | 16MB     |       9 | 1.28
   5 |          ->  Streaming(type: BROADCAST) |     12 | 2MB      |       9 | 1.28
   6 |             ->  Seq Scan on student s   |      4 | 1MB      |       9 | 1.01

 Predicate Information (identified by plan id)
 ---------------------------------------------
   2 --Hash Join (3,4)
         Hash Cond: (ms.id = s.id)

   ====== Query Summary =====
 -------------------------------
 System available mem: 1761280KB
 Query Max mem: 1761280KB
 Query estimated mem: 4400KB
(19 rows)
  • 交叉连接（CROSS JOIN）

    语法：

    1
    		 
    left_table CROSS JOIN right_table

    说明：表示left_table中所有行和right_table中的所有行分别进行连接，最终结果行数等于两边行数的乘积。又称笛卡尔积。

    示例2：学生表和数学成绩表的交叉连接。

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    SELECT s.id, s.name, ms.score FROM student s CROSS JOIN math_score ms;
 id | name  | score
----+-------+-------
  3 | Tina  |    80
  2 | Lily  |    80
  1 | Tom   |    80
  4 | Perry |    80
  3 | Tina  |
  2 | Lily  |
  1 | Tom   |
  4 | Perry |
  3 | Tina  |    95
  2 | Lily  |    95
  1 | Tom   |    95
  4 | Perry |    95
  2 | Lily  |    75
  3 | Tina  |    75
  1 | Tom   |    75
  4 | Perry |    75
(16 rows)

EXPLAIN SELECT s.id, s.name, ms.score FROM student s CROSS JOIN math_score ms;
                                       QUERY PLAN
----------------------------------------------------------------------------------------
  id |                operation                | E-rows | E-memory | E-width | E-costs
 ----+-----------------------------------------+--------+----------+---------+---------
   1 | ->  Streaming (type: GATHER)            |    120 |          |      13 | 19.89
   2 |    ->  Nested Loop (3,4)                |    120 | 1MB      |      13 | 11.89
   3 |       ->  Seq Scan on math_score ms     |     30 | 1MB      |       4 | 10.10
   4 |       ->  Materialize                   |     12 | 16MB     |       9 | 1.30
   5 |          ->  Streaming(type: BROADCAST) |     12 | 2MB      |       9 | 1.28
   6 |             ->  Seq Scan on student s   |      4 | 1MB      |       9 | 1.01

   ====== Query Summary =====
 -------------------------------
 System available mem: 1761280KB
 Query Max mem: 1761280KB
 Query estimated mem: 4144KB
(14 rows)
  • 自然连接（NATURAL JOIN）

    语法：

    1
    		 
    left_table NATURAL JOIN right_table

    说明：表示left_table和right_table中列名相同的列进行等值连接，且自动将同名列只保留一份。

    示例3：学生表和数学成绩表的自然连接，两表的同名列为id列，将按照id列进行等值连接。

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    SELECT * FROM student s NATURAL JOIN math_score ms;
 id | name  | score
----+-------+-------
  1 | Tom   |    80
  4 | Perry |    95
  2 | Lily  |    75
(3 rows)

EXPLAIN SELECT * FROM student s NATURAL JOIN math_score ms;
                                       QUERY PLAN
----------------------------------------------------------------------------------------
  id |                operation                | E-rows | E-memory | E-width | E-costs
 ----+-----------------------------------------+--------+----------+---------+---------
   1 | ->  Streaming (type: GATHER)            |      4 |          |      13 | 19.47
   2 |    ->  Hash Join (3,4)                  |      4 | 1MB      |      13 | 11.47
   3 |       ->  Seq Scan on math_score ms     |     30 | 1MB      |       8 | 10.10
   4 |       ->  Hash                          |     12 | 16MB     |       9 | 1.28
   5 |          ->  Streaming(type: BROADCAST) |     12 | 2MB      |       9 | 1.28
   6 |             ->  Seq Scan on student s   |      4 | 1MB      |       9 | 1.01

 Predicate Information (identified by plan id)
 ---------------------------------------------
   2 --Hash Join (3,4)
         Hash Cond: (ms.id = s.id)

   ====== Query Summary =====
 -------------------------------
 System available mem: 1761280KB
 Query Max mem: 1761280KB
 Query estimated mem: 4400KB
(19 rows)

外连接

  • 左外连接（LEFT JOIN）

    语法：

    1
    		 
    left_table LEFT [OUTER] JOIN right_table [ ON join_condition | USING ( join_column )]

    说明：左外连接的结果集包括left_table的所有行，而不仅是连接列所匹配的行。如果left_table的某行在right_table中没有匹配行，则在相关联的结果集行中输出right_table的列均为空值。

    示例4：学生表和数学成绩表进行左外连接，学生表中id为3的行在结果集中对应的右表数据用NULL填充。

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    SELECT s.id, s.name, ms.score FROM student s LEFT JOIN math_score ms on (s.id = ms.id);
 id | name  | score
----+-------+-------
  3 | Tina  |
  1 | Tom   |    80
  2 | Lily  |    75
  4 | Perry |    95
(4 rows)

EXPLAIN SELECT s.id, s.name, ms.score FROM student s LEFT JOIN math_score ms on (s.id = ms.id);
                                        QUERY PLAN
-------------------------------------------------------------------------------------------
  id |                 operation                  | E-rows | E-memory | E-width | E-costs
 ----+--------------------------------------------+--------+----------+---------+---------
   1 | ->  Streaming (type: GATHER)               |      4 |          |      13 | 10.26
   2 |    ->  Hash Left Join (3, 5)               |      4 | 1MB      |      13 | 2.26
   3 |       ->  Streaming(type: REDISTRIBUTE)    |      4 | 2MB      |       9 | 1.11
   4 |          ->  Seq Scan on student s         |      4 | 1MB      |       9 | 1.01
   5 |       ->  Hash                             |      4 | 16MB     |       8 | 1.11
   6 |          ->  Streaming(type: REDISTRIBUTE) |      4 | 2MB      |       8 | 1.11
   7 |             ->  Seq Scan on math_score ms  |      4 | 1MB      |       8 | 1.01

 Predicate Information (identified by plan id)
 ---------------------------------------------
   2 --Hash Left Join (3, 5)
         Hash Cond: (s.id = ms.id)

   ====== Query Summary =====
 ------------------------------
 System available mem: 901120KB
 Query Max mem: 901120KB
 Query estimated mem: 7520KB
(20 rows)
  • 右外连接（RIGHT JOIN）

    语法：

    1
    		 
    left_table RIGHT [OUTER] JOIN right_table [ ON join_condition | USING ( join_column )]

    说明：与左外连接相反，右外连接的结果集包括right_table的所有行，而不仅是连接列所匹配的行。如果right_table的某行在left_table中没有匹配行，则在相关联的结果集行中left_table的列均为空值。

    示例5：学生表和数学成绩表进行右外连接，数学成绩表中id为6的行在结果集中对应的左表数据用NULL填充。

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    SELECT ms.id, s.name, ms.score FROM student s RIGHT JOIN math_score ms on (s.id = ms.id);
 id | name  | score
----+-------+-------
  1 | Tom   |    80
  6 |       |
  4 | Perry |    95
  2 | Lily  |    75

EXPLAIN SELECT ms.id, s.name, ms.score FROM student s RIGHT JOIN math_score ms on (s.id = ms.id);
                                       QUERY PLAN
----------------------------------------------------------------------------------------
  id |                operation                | E-rows | E-memory | E-width | E-costs
 ----+-----------------------------------------+--------+----------+---------+---------
   1 | ->  Streaming (type: GATHER)            |     30 |          |      13 | 19.47
   2 |    ->  Hash Left Join (3, 4)            |     30 | 1MB      |      13 | 11.47
   3 |       ->  Seq Scan on math_score ms     |     30 | 1MB      |       8 | 10.10
   4 |       ->  Hash                          |     12 | 16MB     |       9 | 1.28
   5 |          ->  Streaming(type: BROADCAST) |     12 | 2MB      |       9 | 1.28
   6 |             ->  Seq Scan on student s   |      4 | 1MB      |       9 | 1.01

 Predicate Information (identified by plan id)
 ---------------------------------------------
   2 --Hash Left Join (3, 4)
         Hash Cond: (ms.id = s.id)

   ====== Query Summary =====
 -------------------------------
 System available mem: 1761280KB
 Query Max mem: 1761280KB
 Query estimated mem: 5424KB
(19 rows)

    对于右外连接，在join算子中却显示的是Left。这是因为，右外连接其实就是将左右表进行交互后的左外连接，所以数据库内部实现为了减少处理逻辑，会将右外连接转为左外连接。

  • 全外连接（FULL JOIN）

    语法：

    1
    		 
    left_table FULL [OUTER] JOIN right_table [ ON join_condition | USING ( join_column )]

    说明：全外连接是左外连接和右外连接的综合。全外连接的结果集包括left_table和right_table的所有行，而不仅是连接列所匹配的行。如果left_table的某行在right_table中没有匹配行，则在相关联的结果集行中right_table的列均为空值。如果right_table的某行在left_table中没有匹配行，则在相关联的结果集行中left_table的列均为空值。

    示例6：学生表和数学成绩表进行全外连接，学生表中id为3的行在结果集中对应的右表数据用NULL填充，数学成绩表中id为6的行在结果集中对应的左表数据用NULL填充。

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    SELECT s.id, s.name, ms.id, ms.score FROM student s FULL JOIN math_score ms ON (s.id = ms.id);
 id | name  | id | score
----+-------+----+-------
  2 | Lily  |  2 |    75
  4 | Perry |  4 |    95
  1 | Tom   |  1 |    80
  3 | Tina  |    |
    |       |  6 |
(5 rows)

EXPLAIN SELECT s.id, s.name, ms.id, ms.score FROM student s FULL JOIN math_score ms ON (s.id = ms.id);
                                        QUERY PLAN
-------------------------------------------------------------------------------------------
  id |                 operation                  | E-rows | E-memory | E-width | E-costs
 ----+--------------------------------------------+--------+----------+---------+---------
   1 | ->  Streaming (type: GATHER)               |     30 |          |      17 | 20.24
   2 |    ->  Hash Full Join (3, 5)               |     30 | 1MB      |      17 | 12.24
   3 |       ->  Streaming(type: REDISTRIBUTE)    |     30 | 2MB      |       8 | 11.06
   4 |          ->  Seq Scan on math_score ms     |     30 | 1MB      |       8 | 10.10
   5 |       ->  Hash                             |      4 | 16MB     |       9 | 1.11
   6 |          ->  Streaming(type: REDISTRIBUTE) |      4 | 2MB      |       9 | 1.11
   7 |             ->  Seq Scan on student s      |      4 | 1MB      |       9 | 1.01

 Predicate Information (identified by plan id)
 ---------------------------------------------
   2 --Hash Full Join (3, 5)
         Hash Cond: (ms.id = s.id)

   ====== Query Summary =====
 -------------------------------
 System available mem: 1761280KB
 Query Max mem: 1761280KB
 Query estimated mem: 6496KB
(20 rows)

多表查询中on条件和where条件的区别

从上面各种连接语法中可见，除自然连接和交叉连接外，其他都需要有on条件（using在查询解析过程中会被转为on条件）来限制两表连接的结果。通常在查询的语句中也都会有where条件限制查询结果。这里说的on连接条件和where过滤条件是指不含可以下推到表上的过滤条件。on和where的区别是：

  • on条件是两表连接的约束条件。
  • where是对两表连接后产生的结果集再次进行过滤。

简单总结就是：on条件优先于where条件，在两表进行连接时被应用；生成两表连接结果集后，再应用where条件。