PostgreSql实现mysql的group_concat的聚合操作

group_concat是mysql中的一个聚集函数 ,但是PostgreSql没有这个函数 ,但是可以用string_agg函数来实现

出处: https://blog.csdn.net/u011944141/article/details/78902678

array_agg(expression)
把表达式变成一个数组 一般配合 array_to_string() 函数使用

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string_agg(expression, delimiter)
直接把一个表达式变成字符串

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案例：

create table(empno smallint, ename varchar(20), job varchar(20), mgr smallint, hiredate date, sal bigint, comm bigint, deptno smallint);

insert into jinbo.employee(empno,ename,job, mgr, hiredate, sal, comm, deptno) values (7499, 'ALLEN', 'SALEMAN', 7698, '2014-11-12', 16000, 300, 30);

insert into jinbo.employee(empno,ename,job, mgr, hiredate, sal, comm, deptno) values (7499, 'ALLEN', 'SALEMAN', 7698, '2014-11-12', 16000, 300, 30);

insert into jinbo.employee(empno,ename,job, mgr, hiredate, sal, comm, deptno) values (7654, 'MARTIN', 'SALEMAN', 7698, '2016-09-12', 12000, 1400, 30);

select * from jinbo.employee;
empno | ename  |   job   | mgr  |  hiredate  |  sal  | comm | deptno
-------+--------+---------+------+------------+-------+------+--------
7499 | ALLEN  | SALEMAN | 7698 | 2014-11-12 | 16000 |  300 |     30
7566 | JONES  | MANAGER | 7839 | 2015-12-12 | 32000 |    0 |     20
7654 | MARTIN | SALEMAN | 7698 | 2016-09-12 | 12000 | 1400 |     30
(3 rows)

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1.查询同一个部门下的员工且合并起来

方法1：
select deptno, string_agg(ename, ',') from jinbo.employee group by deptno;

deptno |  string_agg
--------+--------------
20 | JONES
30 | ALLEN,MARTIN

方法2：
select deptno, array_to_string(array_agg(ename),',') from jinbo.employee group by deptno;
deptno | array_to_string
--------+-----------------
20 | JONES
30 | ALLEN,MARTIN

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2、在1条件的基础上，按ename 倒叙合并

select deptno, string_agg(ename, ',' order by ename desc) from jinbo.employee group by deptno;
deptno |  string_agg
--------+--------------
20 | JONES
30 | MARTIN,ALLEN

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3、按数组格式输出使用 array_agg

select deptno, array_agg(ename) from jinbo.employee group by deptno;
deptno |   array_agg
--------+----------------
20 | {JONES}
30 | {ALLEN,MARTIN}

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4、array_agg 去重元素，例如查询所有的部门

select array_agg(distinct deptno) from jinbo.employee;
array_agg
-----------
{20,30}
(1 row)

#不仅可以去重，还可以排序

select array_agg(distinct deptno order by deptno desc) from jinbo.employee;
array_agg
-----------
{30,20}
(1 row)