270 lines
7.7 KiB
Markdown
270 lines
7.7 KiB
Markdown
聚集函数
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=========================
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聚合函数对一组值执行计算并返回单一的值。聚合函数忽略空值。聚合函数经常与SELECT语句的GROUP BY子句一同使用。
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所有聚合函数都具有确定性。任何时候用一组给定的输入值调用它们时，都返回相同的值。
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在OceanBase的聚合函数中，Value表达式只能出现一个。例如：不支持COUNT(c1, c2)，仅支持COUNT(c1）。
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AVG
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------------
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**声明**
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`AVG(([DISTINCT] expr)`
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**说明**
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返回指定组中的平均值，空值被忽略。DISTINCT选项可用于返回expr的不同值的平均值。若找不到匹配的行，则AVG()返回NULL。
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**例子**
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```javascript
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Oceanbase>select * from oceanbasetest;
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+----+------+------+
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| id | ip | ip2 |
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+----+------+------+
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| 1 | 4 | NULL |
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| 3 | 3 | NULL |
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| 4 | 3 | NULL |
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+----+------+------+
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3 rows in set (0.01 sec)
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Oceanbase>select avg(ip2), avg(ip), avg(distinct(ip)) from oceanbasetest;
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+----------+---------+-------------------+
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| avg(ip2) | avg(ip) | avg(distinct(ip)) |
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+----------+---------+-------------------+
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| NULL | 3.3333 | 3.5000 |
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+----------+---------+-------------------+
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1 row in set (0.00 sec)
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Oceanbase>select avg(distinct(ip)),avg(ip),avg(ip2) from oceanbasetest;
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+-------------------+---------+----------+
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| avg(distinct(ip)) | avg(ip) | avg(ip2) |
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+-------------------+---------+----------+
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| 3.5000 | 3.3333 | NULL |
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+-------------------+---------+----------+
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1 row in set (0.00 sec)
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```
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COUNT
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--------------
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**声明**
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`COUNT([DISTINCT] expr)`
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**说明**
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COUNT(\[DISTINCT\] expr )返回SELECT语句检索到的行中非NULL值的数目。若找不到匹配的行，则COUNT()返回0。DISTINCT选项可用于返回 expr 的不同值的数目。
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COUNT(\*)的稍微不同之处在于，它返回检索行的数目，不论其是否包含NULL值。
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**例子**
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```javascript
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Oceanbase>select * from oceanbasetest;
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+----+------+------+
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| id | ip | ip2 |
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+----+------+------+
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| 1 | 4 | NULL |
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| 3 | 3 | NULL |
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| 4 | 3 | NULL |
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+----+------+------+
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3 rows in set (0.00 sec)
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Oceanbase>select count(ip2), count(ip), count(distinct(ip)), count(*) from oceanbasetest;
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+------------+-----------+---------------------+----------+
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| count(ip2) | count(ip) | count(distinct(ip)) | count(*) |
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+------------+-----------+---------------------+----------+
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| 0 | 3 | 2 | 3 |
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+------------+-----------+---------------------+----------+
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1 row in set (0.00 sec)
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```
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MAX
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------------
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**声明**
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`MAX([DISTINCT] expr)`
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**说明**
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返回指定数据中的最大值。
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MAX()的取值可以是一个字符串参数；在这些情况下，它们返回最大字符串值。DISTINCT关键字可以被用来查找 expr的不同值的最大值，这产生的结果与省略DISTINCT 的结果相同。
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假设表a有三行数据：id=1，num=10；id=2，num=20；id=3，num=30。
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**例子**
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```javascript
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Oceanbase>SELECT MAX(num) FROM a;
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+-----------------+
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| MAX(num) |
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+-----------------+
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| 30 |
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+-----------------+
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1 row in set (0.00 sec)
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```
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MIN
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------------
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**声明**
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`MIN([DISTINCT] expr)`
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**说明**
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返回指定数据中的最小值。
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MIN()的取值可以是一个字符串参数；在这些情况下，它们返回最小字符串值。DISTINCT关键字可以被用来查找expr 的不同值的最小值，然而，这产生的结果与省略DISTINCT 的结果相同。
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假设表a有三行数据：id=1，num=10；id=2，num=20；id=3，num=30。
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**例子**
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```javascript
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Oceanbase>SELECT MIN(num) FROM a;
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+----------------+
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| MIN(num) |
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+----------------+
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| 10 |
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+----------------+
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1 row in set (0.00 sec)
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```
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SUM
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------------
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**声明**
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`SUM([DISTINCT] expr)`
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**说明**
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返回 expr 的总数。若返回集合中无任何行，则 SUM() 返回NULL。DISTINCT关键字可用于求得 expr 不同值的总和。
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若找不到匹配的行，则SUM()返回NULL。
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**例子**
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```javascript
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Oceanbase>select * from oceanbasetest;
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+------+------+------+
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| id | ip | ip2 |
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+------+------+------+
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| 1 | 4 | NULL |
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| 3 | 3 | NULL |
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| 4 | 3 | NULL |
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+------+------+------+
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3 rows in set (0.00 sec)
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Oceanbase>select sum(ip2),sum(ip),sum(distinct(ip)) from oceanbasetest;
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+----------+---------+-------------------+
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| sum(ip2) | sum(ip) | sum(distinct(ip)) |
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+----------+---------+-------------------+
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| NULL | 10 | 7 |
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+----------+---------+-------------------+
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1 row in set (0.00 sec)
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```
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GROUP_CONCAT
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---------------------
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**声明**
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`GROUP_CONCAT([DISTINCT] expr)`
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**说明**
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该函数返回带有来自一个组的连接的非NULL值的字符串结果。
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```javascript
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GROUP_CONCAT([DISTINCT] expr [,expr ...]
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[ORDER BY {unsigned_integer | col_name | expr}
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ASC | DESC]
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[SEPARATOR str_val])
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```
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**例子**
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```javascript
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Oceanbase>select * from book; //表book（书编号，书名，出版社）
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+--------+--------------------------------+-----------------------------+
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| bookid | bookname | publishname |
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+--------+--------------------------------+-----------------------------+
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| 1 | git help | alibaba group publisher |
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| 2 | MySQL性能优化 | 浙江大学图文出版社 |
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| 3 | JAVA编程指南 | 机械工业出版社 |
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| 3 | JAVA编程指南 | 机械工业出版社 |
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| 4 | 大规模分布式存储系统 | 机械工业出版社 |
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+--------+--------------------------------+-----------------------------+
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5 rows in set (0.00 sec)
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//查找书名信息
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Oceanbase>select group_concat(bookname) from book group by bookname;
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+-----------------------------------+
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| group_concat(bookname) |
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+-----------------------------------+
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| git help |
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| JAVA编程指南,JAVA编程指南 |
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| MySQL性能优化 |
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| 大规模分布式存储系统 |
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+-----------------------------------+
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4 rows in set (0.00 sec)
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//查找书名信息，书名唯一
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Oceanbase>select group_concat(distinct(bookname)) from book group by bookname;
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+----------------------------------+
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| group_concat(distinct(bookname)) |
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+----------------------------------+
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| git help |
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| JAVA编程指南 |
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| MySQL性能优化 |
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| 大规模分布式存储系统 |
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+----------------------------------+
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4 rows in set (0.01 sec)
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//查找书名和出版社信息，以书名分组，出版社信息降序排序显示
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Oceanbase>select bookname, group_concat(publishname order by publishname desc separator ';' ) from book group by bookname;
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+--------------------------------+---------------------------------------------------------------------+
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| bookname | group_concat(publishname order by publishname desc separator ';' ) |
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+--------------------------------+---------------------------------------------------------------------+
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| git help | alibaba group publisher |
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| JAVA编程指南 | 机械工业出版社;机械工业出版社 |
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| MySQL性能优化 | 浙江大学图文出版社 |
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| 大规模分布式存储系统 | 机械工业出版社 |
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+--------------------------------+---------------------------------------------------------------------+
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4 rows in set (0.00 sec)
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```
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