377 lines
12 KiB
Markdown
377 lines
12 KiB
Markdown
SELECT
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===========================
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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。
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SIMPLE SELECT
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----------------------
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### 描述
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该语句用于查询表中的内容。
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### 格式
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```javascript
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simple_select:
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SELECT [/*+ hint statement */] [ALL | DISTINCT | UNIQUE | SQL_CALC_FOUND_ROWS]
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select_expr_list FROM from_list [WHERE condition]
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[GROUP BY group_expression_list [WITH ROLLUP] [HAVING condition]]
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[ORDER BY order_expression_list]
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[limit_clause]
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[FOR UPDATE]
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select_expr:
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table_name.*
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| table_alias_name.*
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| expr [[AS] column_alias_name]
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from_list:
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table_reference [, table_reference ...]
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table_reference:
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simple_table
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| joined_table
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simple_table:
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table_factor [partition_option] [[AS] table_alias_name]
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| (select_stmt) [AS] table_alias_name
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| (table_reference_list)
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joined_table:
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table_reference [INNER] JOIN simple_table [join_condition]
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| table_reference outer_join_type JOIN simple_table join_condition
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partition_option:
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PARTITION (partition_name_list)
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partition_name_list:
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partition_name [, partition_name ...]
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outer_join_type:
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{LEFT | RIGHT | FULL} [OUTER]
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join_condition:
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ON expression
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condition:
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expression
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group_expression_list:
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group_expression [, group_expression ...]
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group_expression:
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expression [ASC | DESC]
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order_expression_list:
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order_expression [, order_expression ...]
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order_expression:
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expression [ASC | DESC]
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limit_clause:
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LIMIT {[offset,] row_count |row_count OFFSET offset}
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```
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### 参数解释
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| 参数 | 描述 |
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|--------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| ALL \| DISTINCT \| UNIQUE \| SQL_CALC_FOUND_ROWS | 在数据库表中，可能会包含重复值。 * 指定"DISTINCT"，则在查询结果中相同的行只显示一行； * 指定"ALL"，则列出所有的行； * 指定"SQL_CALC_FOUND_ROWS"，则不输出数据，只返回数据行数； * 不指定时，默认为"ALL"。 |
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| select_expr | 列出要查询的表达式或列名，用","隔开。也可以用"\*"表示所有列。 |
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| AS othername | 为输出字段重新命名。 |
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| FROM table_references | 指名了从哪个表或哪些表中读取数据（支持多表查询）。 |
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| WHERE where_conditions | 可选项，WHERE 字句用来设置一个筛选条件，查询结果中仅包含满足条件的数据。where_conditions 为表达式。 |
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| GROUP BY group_by_list | 用于进行分类汇总。 |
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| HAVING search_confitions | HAVING 字句与 WHERE 字句类似，但是 HAVING 字句可以使用累计函数（如 SUM，AVG 等）。 |
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| ORDER BY order_list order_list : colname \[ASC \| DESC\] \[,colname \[ASC \| DESC\]...\] | 用来按升序（ASC）或者降序（DESC）显示查询结果。不指定 ASC 或者 DESC 时，默认为 ASC。 |
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| \[LIMIT {\[offset,\] row_count \|row_count OFFSET offset}\] | 强制 SELECT 语句返回指定的记录数。 LIMIT 接受一个或两个数字参数。参数必须是一个整数常量。 * 如果给定两个参数，第一个参数指定第一个返回记录行的偏移量，第二个参数指定返回记录行的最大数目。初始记录行的偏移量是 0(而不是 1)。 * 如果只给定一个参数，它表示返回记录行的最大数目，偏移量为0。 |
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| FOR UPDATE | 对查询结果所有行上排他锁，以阻止其他事务的并发修改，或阻止在某些事务隔离级别时的并发读取。 |
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| PARTITION(partition_list) | 指定查询表的分区信息。例如：partition(p0,p1...) |
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### 示例
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以如下表 a 为例。
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* 从表 a 中读取 name 数据。
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```javascript
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SELECT name FROM a;
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```
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* 在查询结果中对 name 进行去重处理。
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```javascript
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SELECT DISTINCT name FROM a;
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```
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* 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。
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```javascript
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SELECT id, name, num/2 AS avg FROM a;
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```
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* 从表 a 中根据筛选条件" name = 'a' " ，输出对应的 id 、name 和 num 。
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```javascript
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SELECT id, name, num FROM a WHERE name = 'a';
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```
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* 从表 a 中查询 id，name，按照 name 分组对 num 求和，并输出。
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```javascript
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SELECT id, name, SUM(num) FROM a GROUP BY name;
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```
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* 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。
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```javascript
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SELECT id, name, SUM(num) as sum FROM a GROUP BY name HAVING SUM(num) < 160;
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```
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* 从表 a 中查询 id，name，num，根据 num 按升序（ASC）输出查询结果。
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```javascript
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SELECT * FROM a ORDER BY num ASC;
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```
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* 从表 a 中查询 id，name，num，根据 num 按降序（DESC）输出查询结果。
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```javascript
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SELECT * FROM a ORDER BY num DESC;
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```
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* 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。
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```javascript
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SELECT * FROM a LIMIT 1,2;
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```
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集合类 SELECT
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-------------------
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### 描述
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该语句用于对多个 SELECT 查询的结果进行 UNION，MINUS，INTERSECT。
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### 格式
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```javascript
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select_clause_set:
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simple_select [ UNION | UNION ALL | EXCEPT | INTERSECT] select_clause_set_left
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[ORDER BY sort_list_columns] [limit_clause]
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select_clause_set_right:
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simple_select |
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select_caluse_set
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```
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### 参数解释
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| 参数 | 描述 |
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|-----------|--------------------------|
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| UNION ALL | 合并两个查询的结果 |
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| UNION | 合并两个查询的结果，并去重 |
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| EXCEPT | 从左查询结果集中去重出现在右查询中的结果，并去重 |
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| INTERSECT | 保留左查询结果集中出现在右查询中的结果，并去重 |
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### 示例
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以如下两表的数据为例：
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```javascript
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create table t1 (c1 int, c2 int);
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create table t2 (c1 int, c2 int);
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insert into t1 values (1, -1), (2, -2);
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insert into t2 values (1, 1), (2, -2), (3, 3);
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```
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* 计算 T1, T2 的所有的记录
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```javascript
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SELECT C1, C2 FROM T1 UNION ALL SELECT C1, C2 FROM T2;
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+------+------+
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| C1 | C2 |
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+------+------+
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| 1 | -1 |
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| 2 | -2 |
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| 1 | 1 |
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| 2 | -2 |
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| 3 | 3 |
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+------+------+
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```
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* 计算 T1, T2 的去重后的所有记录
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```javascript
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SELECT C1, C2 FROM T1 UNION SELECT C1, C2 FROM T2;
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+------+------+
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| C1 | C2 |
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+------+------+
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| 1 | -1 |
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| 2 | -2 |
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| 1 | 1 |
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| 3 | 3 |
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+------+------+
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```
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* 计算 T1 和 T2 的交集
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```javascript
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SELECT C1, C2 FROM T1 INTERSECT SELECT C1, C2 FROM T2;
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+------+------+
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| C1 | C2 |
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+------+------+
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| 2 | -2 |
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+------+------+
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```
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* 计算 T1 和 T2 的差集
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```javascript
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SELECT C1, C2 FROM T1 EXCEPT SELECT C1, C2 FROM T2;
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+------+------+
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| C1 | C2 |
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+------+------+
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| 1 | -1 |
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+------+------+
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```
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* 取 T1 和 T2 并集中 C2 排序最大的前两行
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```javascript
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SELECT C1, C2 FROM T1 UNION SELECT C1, C2 FROM T2 ORDER BY C2 DESC LIMIT 2;
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+------+------+
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| C1 | C2 |
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+------+------+
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| 3 | 3 |
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| 1 | 1 |
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+------+------+
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```
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