OceanBase SELECT

SELECT 的语法相对比较复杂。本节首先会介绍普通的 SELECT 语法结构，然后介绍集合类 SELECT 的语法结构。

SIMPLE SELECT

描述

该语句用于查询表中的内容。

格式

simple_select:
SELECT [] [ALL | DISTINCT | UNIQUE | SQL_CALC_FOUND_ROWS] 
 select_expr_list FROM from_list [WHERE condition] 
  [GROUP BY group_expression_list [WITH ROLLUP] [HAVING condition]]
  [ORDER BY order_expression_list]
  [limit_clause]
  [FOR UPDATE]

select_expr:
 table_name.*
  | table_alias_name.*
  | expr [[AS] column_alias_name]
  
from_list:
    table_reference [, table_reference ...]

table_reference:
      simple_table
    | joined_table

simple_table:
    table_factor [partition_option] [[AS] table_alias_name]
    | (select_stmt) [AS] table_alias_name
    | (table_reference_list)

joined_table:
      table_reference [INNER] JOIN simple_table [join_condition]
    | table_reference outer_join_type JOIN simple_table join_condition

partition_option:
    PARTITION (partition_name_list)

partition_name_list:
    partition_name [, partition_name ...]

outer_join_type:
    {LEFT | RIGHT | FULL} [OUTER]

join_condition:
    ON expression

condition:
    expression

group_expression_list:
    group_expression [, group_expression ...]

group_expression:
    expression [ASC | DESC]

order_expression_list:
    order_expression [, order_expression ...]

order_expression:
    expression [ASC | DESC]
    
limit_clause:
  LIMIT {[offset,] row_count |row_count OFFSET offset}

参数解释

示例

以如下表 a 为例。

• 从表 a 中读取 name 数据。

obclient> SELECT name FROM a;
+------+
| name |
+------+
| a    |
| b    |
| a    |
+------+
3 rows in set (0.01 sec)

• 在查询结果中对 name 进行去重处理。

obclient> SELECT DISTINCT name FROM a;
+------+
| name |
+------+
| a    |
| b    |
+------+
2 rows in set (0.01 sec)

• 从表 a 中查询 id，name，num，然后把 num 列除以 2 输出，输出的列名为 avg。

obclient> SELECT id, name, num/2 AS avg FROM a;
+------+------+----------+
| id   | name | avg      |
+------+------+----------+
|    1 | a    |  50.0000 |
|    2 | b    | 100.0000 |
|    3 | a    |  25.0000 |
+------+------+----------+
3 rows in set (0.01 sec)

• 从表 a 中根据筛选条件“ name = 'a' ” ，输出对应的 id 、name 和 num 。

obclient> SELECT id, name, num FROM a WHERE name = 'a';
+------+------+------+
| id   | name | num  |
+------+------+------+
|    1 | a    |  100 |
|    3 | a    |   50 |
+------+------+------+
2 rows in set (0.01 sec)

• 从表 a 中查询 id，name，按照 name 分组对 num 求和，并输出。

obclient> SELECT id, name, SUM(num) FROM a GROUP BY name;
+------+------+----------+
| id   | name | SUM(num) |
+------+------+----------+
|    1 | a    |      150 |
|    2 | b    |      200 |
+------+------+----------+
2 rows in set (0.00 sec)

• 从表 a 中查询 id，name，按照 name 分组对 num 求和，查询 num 总和小于 160 的行，并输出。

obclient> SELECT id, name, SUM(num) as sum FROM a GROUP BY name HAVING SUM(num) < 160;
+------+------+------+
| id   | name | sum  |
+------+------+------+
|    1 | a    |  150 |
+------+------+------+
1 row in set (0.01 sec)

• 从表 a 中查询 id，name，num，根据 num 按升序（ASC）输出查询结果。

obclient> SELECT * FROM a ORDER BY num ASC;
+------+------+------+
| id   | name | num  |
+------+------+------+
|    3 | a    |   50 |
|    1 | a    |  100 |
|    2 | b    |  200 |
+------+------+------+
3 rows in set (0.00 sec)

• 从表 a 中查询 id，name，num，根据 num 按降序（DESC）输出查询结果。

obclient> SELECT * FROM a ORDER BY num DESC;
+------+------+------+
| id   | name | num  |
+------+------+------+
|    2 | b    |  200 |
|    1 | a    |  100 |
|    3 | a    |   50 |
+------+------+------+
3 rows in set (0.00 sec)

• 从表 a 中查询 id，name，num，使用 LIMIT强制从第2行开始，返回表 a 的两行结果。

obclient> SELECT * FROM a LIMIT 1,2;
+------+------+------+
| id   | name | num  |
+------+------+------+
|    2 | b    |  200 |
|    3 | a    |   50 |
+------+------+------+
2 rows in set (0.00 sec)​

集合类 SELECT

描述

该语句用于对多个 SELECT 查询的结果进行 UNION，MINUS，INTERSECT。

格式

select_clause_set:
    simple_select [ UNION | UNION ALL | EXCEPT | INTERSECT] select_clause_set_left 
  [ORDER BY sort_list_columns] [limit_clause]
select_clause_set_right:
    simple_select | 
  select_caluse_set

参数解释

示例

以如下两表的数据为例：

create table t1 (c1 int, c2 int);
create table t2 (c1 int, c2 int);
insert into t1 values (1, -1), (2, -2);
insert into t2 values (1, 1), (2, -2), (3, 3);

• 计算 T1, T2 的所有的记录

obclient> SELECT C1, C2 FROM T1 UNION ALL SELECT C1, C2 FROM T2;
+------+------+
| C1   | C2   |
+------+------+
|    1 |   -1 |
|    2 |   -2 |
|    1 |    1 |
|    2 |   -2 |
|    3 |    3 |
+------+------+
5 rows in set (0.01 sec)

• 计算 T1, T2 的去重后的所有记录

obclient> SELECT C1, C2 FROM T1 UNION SELECT C1, C2 FROM T2;
+------+------+
| C1   | C2   |
+------+------+
|    1 |   -1 |
|    2 |   -2 |
|    1 |    1 |
|    3 |    3 |
+------+------+
4 rows in set (0.01 sec)

• 计算 T1 和 T2 的交集

obclient> SELECT C1, C2 FROM T1 INTERSECT SELECT C1, C2 FROM T2;
+------+------+
| C1   | C2   |
+------+------+
|    2 |   -2 |
+------+------+
1 row in set (0.00 sec)

• 计算 T1 和 T2 的差集

obclient> SELECT C1, C2 FROM T1 EXCEPT SELECT C1, C2 FROM T2;
+------+------+
| C1   | C2   |
+------+------+
|    1 |   -1 |
+------+------+
1 row in set (0.00 sec)

• 取 T1 和 T2 并集中 C2 排序最大的前两行

obclient> SELECT C1, C2 FROM T1 UNION SELECT C1, C2 FROM T2 ORDER BY C2 DESC LIMIT 2;
+------+------+
| C1   | C2   |
+------+------+
|    3 |    3 |
|    1 |    1 |
+------+------+
2 rows in set (0.00 sec)