如何使用SQL从数据库中获取键值对

Question

I'm a new to SQL so, can you please help me in figuring out how to get a key-value pair from a SQL for example : Table A

A(PK)    B(INT)  C       D
------ ------- ------- --------

1        X       Y        Z

2        A       B        C

I want the output in the form

A = 1
B = X 
C = Y 
D = Z

i tried

SELECT A,B,C,D from tableA ; 

But it only prints in the form 1 XYZ So, can you please help me ?

Answer 1

Generally getting the column names along with the row values and formatting the result is a thing the SQL client provides for you. pgAdmin isn't really suited for this, a programming language would be better.

For example, here's how you'd do it using Ruby and the ruby-pg gem .

#!/usr/bin/env ruby

require 'pg'

conn = PG.connect( dbname: 'test' )
conn.exec( "SELECT * FROM tableA" ) do |result|
  result.each do |row|
    row.each do |column,value|
      puts "#{column} = #{value}"
    end
  end
end

Answer 2

You can do a union of the queries picking one column at a time

SELECT 'A', A from tableA WHERE ID = @ID
UNION
SELECT 'B', B from tableA WHERE ID = @ID
UNION
SELECT 'C', C from tableA WHERE ID = @ID
UNION
SELECT 'D', D from tableA WHERE ID = @ID

you will probably need to cast all the columns to a common type (varchar) for this to work.

Answer 3

Another option is to create a function that returns the result as desired:

create type crosstab_type as (row_num bigint, column_name text, value text);

create or replace function kv()
  returns setof crosstab_type
as
$$
declare 
  l_rec record;
  l_row bigint := 1;
begin
  for l_rec in select * from ani 
  loop
    return next (l_row, 'a', l_rec.a::text)::crosstab_type;
    return next (l_row, 'b', l_rec.b::text)::crosstab_type;
    return next (l_row, 'c', l_rec.c::text)::crosstab_type;
    return next (l_row, 'd', l_rec.d::text)::crosstab_type;
    l_row := l_row + 1;
  end loop;
end;
$$
language plpgsql;



select *
from kv();

returns:

row_num | column_name | value
--------+-------------+------
      1 | a           | 1    
      1 | b           | X    
      1 | c           | Y    
      1 | d           | Z    
      2 | a           | 2    
      2 | b           | A    
      2 | c           | B    
      2 | d           | C   

Online example: http://rextester.com/RCKB51862