后浪云OceanBase教程：OceanBase WMSYS.WM_CONCAT/WM_CONCAT

WM_CONCAT 函数用于列转行，将度量列的值进行合并。作为单个集合的聚合函数，WM_CONCAT 对所有行进行操作并返回单个输出行。作为组集聚合，WM_CONCAT 将对 GROUP BY 子句定义的每个组进行操作并返回输出行。作为分析函数，WM_CONCAT 基于 query_partition_clause 中的一个或多个表达式将查询结果集分为几组。

语法

WM_CONCAT ([DISTINCT] measure_expr) [OVER query_partition_clause]

作为分析函数使用时，您需要使用窗口函数的完整语法，它对一组行的集合进行计算并返回多个值。作为聚合函数使用时，该函数对一组行的集合进行聚合计算，结果只能返回一个值，此时不需要加 OVER 关键字。

参数

返回类型

返回的数据类型为 CLOB。

示例

分析函数示例

建表 employees，并向里面插入数据，执行以下语句：

CREATE TABLE employees (department_id INT,manager_id INT,last_name varchar(50),hiredate varchar(50),SALARY INT);
INSERT INTO employees VALUES(30, 100, 'Raphaely', '2017-07-01', 1700);
INSERT INTO employees VALUES(30, 100, 'De Haan', '2018-05-01',11000);      
INSERT INTO employees VALUES(40, 100, 'Errazuriz', '2017-07-21', 1400);
INSERT INTO employees VALUES(50, 100, 'Hartstein', '2019-05-01',14000);     
INSERT INTO employees VALUES(50, 100, 'Raphaely', '2017-07-22', 1700);
INSERT INTO employees VALUES(70, 100, 'Weiss',  '2019-07-11',13500);     
INSERT INTO employees VALUES(90, 100, 'Russell', '2019-10-05', 13000);
INSERT INTO employees VALUES(90,100, 'Partners',  '2018-12-01',14000);

查询 2019 年 10 月 10 日之前雇用的雇员，以及该雇员的部门，雇用日期以及该部门中的其他雇员。执行以下语句：

SELECT department_id "Dept", hiredate "Date", last_name "Name",wmsys.wm_concat(last_name) 
 OVER (PARTITION BY department_id) as "Emp_list"
FROM employees WHERE hiredate < '2019-10-10' ORDER BY "Dept", "Date", "Name";

查询结果如下：

+------+------------+-----------+--------------------+
| Dept | Date       | Name      | Emp_list           |
+------+------------+-----------+--------------------+
|   30 | 2017-07-01 | Raphaely  | Raphaely,De Haan   |
|   30 | 2018-05-01 | De Haan   | Raphaely,De Haan   |
|   40 | 2017-07-21 | Errazuriz | Errazuriz          |
|   50 | 2017-07-22 | Raphaely  | Hartstein,Raphaely |
|   50 | 2019-05-01 | Hartstein | Hartstein,Raphaely |
|   70 | 2019-07-11 | Weiss     | Weiss              |
|   90 | 2018-12-01 | Partners  | Russell,Partners   |
|   90 | 2019-10-05 | Russell   | Russell,Partners   |
+------+------------+-----------+--------------------+
8 rows in set (0.01 sec)

聚合函数示例

建表 employees，并向里面插入数据，执行以下语句：

CREATE TABLE employees (department_id INT,manager_id INT,last_name varchar(50),hiredate varchar(50),SALARY INT);
INSERT INTO employees VALUES(30, 100, 'Raphaely', '2017-07-01', 1700);
INSERT INTO employees VALUES(30, 100, 'De Haan', '2018-05-01',11000);      
INSERT INTO employees VALUES(30, 100, 'Errazuriz', '2017-07-01', 1400);
INSERT INTO employees VALUES(30, 100, 'Hartstein', '2019-05-01',14000);     
INSERT INTO employees VALUES(30, 100, 'Raphaely', '2017-07-01', 1700);
INSERT INTO employees VALUES(30, 100, 'Weiss',  '2019-07-01',13500);     
INSERT INTO employees VALUES(30, 100, 'Russell', '2019-07-01', 13000);
INSERT INTO employees VALUES(30,100, 'Partners',  '2018-12-01',14000);

查询第 30 部门的所有员工，并查找最早的雇用日期。执行以下语句：

SELECT wmsys.wm_concat(last_name) as "Emp_list", MIN(hiredate) as "Earliest"
 FROM employees WHERE department_id = 30;

查询结果如下：

+----------------------------------------------------------------------+------------+
| Emp_list                                                             | Earliest   |
+----------------------------------------------------------------------+------------+
| Raphaely,De Haan,Errazuriz,Hartstein,Raphaely,Weiss,Russell,Partners | 2017-07-01 |
+----------------------------------------------------------------------+------------+
1 row in set (0.01 sec)