Group concat is an aggregation function in mysql, but PostgreSql does not have this function, but it can be implemented with string AGG function

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

array_agg(expression)
To change an expression into an array is generally used in conjunction with the array'to'string() function

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string_agg(expression, delimiter)
Turn an expression directly into a string

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Case study:

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. Query and consolidate employees in the same department

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

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

//Method 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. On the basis of 1 condition, reverse merge by ename

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. Output in array format using array

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 ABCD de duplication elements, such as querying all departments

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

#Not only can they be de duplicated, but they can also be sorted

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