SELECT multiple values from one table having matching record in another table in one row

Question

I have two tables as follows:

create table first (id int, name varchar(20));
create table second (id int, value char(1),fid int); 

These tables have the following records:

insert into first values (1,'Ahmad'),(2,'Sami'),(3,'Khan');
insert into second values
 (1,'a',1)
, (2,'b',1)
, (3,'c',2)
, (4,'d',1)
, (5,'e',2)
, (6,'f',3); 

I want to select those records from table second which have matching records in table first in a single row.

The desired output would be:

 id | name | value | value | value
 ---+-------+-------+-------+-------
 1 | Ahmad | a | b | d
 2 | Sami | c | e |
 3 | Khan | f | | 

The number of values for a single record of table first is not limited and there can be n number of values for a single record of table first.

n is unknown, but I need to repeat the column value for each value in table second which has matching record in table first. I need the whole table like above The desired result. I need only those rows referenced by table second. The version of Postgres will be the latest one.

Answer 1

Actually, since

there can be n number of values for a single record of table first.

the number of columns to return is not exactly arbitrary. There is a maximum of columns, and this has a clean solution - unless you have more columns than Postgres allows for a table:

250 - 1600 depending on column types

BTW, typically, you would also have a third table listing all possible values of value, the whole of it implementing a classical many-to-many relationship.

• How to implement a many-to-many relationship in PostgreSQL?

You can use CASE statements, or more elegantly, the crosstab() function of the additional module tablefunc. If you are unfamiliar with it, read basic instructions here first:

• PostgreSQL Crosstab Query

Columns to the right of actual values are filled with NULL. Assuming a maximum of 5 possible values and building on this setup:

CREATE TABLE first (id int, name text);
CREATE TABLE second (id int, value "char", fid int); 
INSERT INTO first VALUES
 (1,'Ahmad')
, (2,'Sami')
, (3,'Khan')
, (4,'Nobody'); -- Added to demonstrate difference
INSERT INTO second VALUES
 (1,'a',1)
, (2,'b',1)
, (3,'c',2)
, (4,'d',1)
, (5,'e',2)
, (6,'f',3);

Either use crosstab(text) (1 parameter form) and join to table first another time:

SELECT id, f.name, value1, value2, value3, value4, value5
FROM crosstab(
 'SELECT f.id, 1 AS dummy_category, s.value
 FROM first f
 JOIN second s ON s.fid = f.id
 ORDER BY f.id, s.value'
 ) ct (id int
 , value1 "char", value2 "char", value3 "char", value4 "char", value5 "char")
JOIN first f USING (id);

Or use crosstab(text, text) (2 parameter form) and generate dummy categories for your values:

SELECT *
FROM crosstab(
 'SELECT f.id, f.name
 , row_number() OVER (PARTITION BY f.id ORDER BY s.value) AS dummy_category
 , s.value
 FROM first f
 JOIN second s ON s.fid = f.id
 ORDER BY f.id, s.value'
 , ('SELECT generate_series(1,5)')
 ) ct (id int, name text
 , value1 "char", value2 "char", value3 "char", value4 "char", value5 "char");

Result is the same either way:

 id | name | value1 | value2 | value3 | value4 | value5
----+-------+--------+--------+--------+--------+--------
 1 | Ahmad | a | b | d | |
 2 | Sami | c | e | | |
 3 | Khan | f | | | |

If you want to include all rows from table first, make it a LEFT [OUTER] JOIN:

 ...
 LEFT JOIN second s ON s.fid = f.id
 ...

Then we get one additional result row for the above example:

 4 | Nobody | | | | |

Answer 2

The thing is this: you can't get an arbitrary number of columns based on values in rows using regular sql.

You can get similar results to what you want by:

Select f.id, f.name, array_agg(s.value)
From first f inner join second s on f.id = s.fid
Group by f.id, f.name

Which will return three columns, the third one will be an array.