PostgreSQL:string_agg 多列值聚合成一列的操作示例

string_agg(expression, delimiter)

SELECT string_agg(name, ', ') AS concatenated_names
FROM employee;

create table employee(
    id int4 primary key,
    name varchar(100)
);
comment on table employee is '职工表';
comment on column employee.name is '职工名';
insert into employee(id,name) values (1,'张三');
insert into employee(id,name) values (2,'李四');
insert into employee(id,name) values (3,'王二');
insert into employee(id,name) values (4,'麻子');
select string_agg(name,', ') as concatenated_names
from employee;

string_agg(expression,delimiter)

CREATE TABLE city(
	country character varying(64),
	city character varying(64)
);
INSERT INTO city VALUES
('中国','台北'),
('中国','香港'),
('中国','上海'),
('日本','东京'),
('日本','大阪');

SELECT country,string_agg(city,',')
FROM city 
GROUP BY country; 

array_agg(expression)  --输入参数为任何非数组类型

-- 创建数组
create table test_array(id int4[]);
INSERT INTO test_array(id) values(array[1,2,4]),(array[4,5,6]);

-- string_to_array 将字符串转成数字，通过自定切割符合。
select array_length(string_to_array('1.2.3.4.5','.'), 1); --5
--array_length(anyarray,int) 返回数组指定维度的长度