create table row2col(name varchar(20),subject varchar(20),grade varchar(10));
insert into row2col SELECT 'Tim' name, '数学' subject, 'A' grade
UNION
SELECT 'Tim' name, '英语' subject, 'B' grade
UNION
SELECT 'Tim' name, '语文' subject, null grade
UNION
SELECT 'Tom' name, '数学' subject, 'B' grade
UNION
SELECT 'Tom' name, '英语' subject, 'D' grade
UNION
SELECT 'Tom' name, '语文' subject, 'B' grade;
行转列：
select t.name,t.grades[1] 数学,t.grades[2] 英语,t.grades[3] 语文 from(
SELECT
t.name,
array_agg(t.grade order by t.subject) grades
FROM row2col t group by t.name)t;
name | 数学 | 英语 | 语文
------+------+------+------
Tim | A | B |
Tom | B | D | B
列转行：
select name,unnest(array['数学','英语','语文']),unnest(array[数学,英语,语文]) from (select t.name,t.grades[1] 数学,t.grades[2] 英语,t.grades[3] 语文 from(
SELECT
t.name,
array_agg(t.grade order by t.subject) grades
FROM row2col t group by t.name)t
)a;
name | unnest | unnest
------+--------+--------
Tim | 数学 | A
Tim | 英语 | B
Tim | 语文 |
Tom | 数学 | B
Tom | 英语 | D
Tom | 语文 | B
2、递归的实现方式
CREATE TABLE employees (
employee_id serial PRIMARY KEY,
full_name VARCHAR NOT NULL,
manager_id INT
);
INSERT INTO employees (
employee_id,
full_name,
manager_id
)
VALUES
(1, 'Michael North', NULL),
(2, 'Megan Berry', 1),
(3, 'Sarah Berry', 1),
(4, 'Zoe Black', 1),
(5, 'Tim James', 1),
(6, 'Bella Tucker', 2),
(7, 'Ryan Metcalfe', 2),
(8, 'Max Mills', 2),
(9, 'Benjamin Glover', 2),
(10, 'Carolyn Henderson', 3),
(11, 'Nicola Kelly', 3),
(12, 'Alexandra Climo', 3),
(13, 'Dominic King', 3),
(14, 'Leonard Gray', 4),
(15, 'Eric Rampling', 4),
(16, 'Piers Paige', 7),
(17, 'Ryan Henderson', 7),
(18, 'Frank Tucker', 8),
(19, 'Nathan Ferguson', 8),
(20, 'Kevin Rampling', 8);
返回id为2的经理的所有下属
WITH RECURSIVE subordinates AS (
SELECT
employee_id,
manager_id,
full_name
FROM
employees
WHERE
employee_id = 2
UNION
SELECT
e.employee_id,
e.manager_id,
e.full_name
FROM
employees e
INNER JOIN subordinates s ON s.employee_id = e.manager_id
) SELECT
*
FROM
subordinates;
employee_id | manager_id | full_name
-------------+------------+-----------------
2 | 1 | Megan Berry
9 | 2 | Benjamin Glover
8 | 2 | Max Mills
7 | 2 | Ryan Metcalfe
6 | 2 | Bella Tucker
19 | 8 | Nathan Ferguson
18 | 8 | Frank Tucker
20 | 8 | Kevin Rampling
16 | 7 | Piers Paige
17 | 7 | Ryan Henderson