Prepend table name to each column in a result set in SQL? (Postgres specifically)

Question

How can I get the label of each column in a result set to prepend the name if its table?

I want this to happen for queries on single tables as well as joins.

Example:

  SELECT first_name, last_name FROM person;

I want the results to be:

 | person.first_name | person.last_name |
 |-------------------|------------------|
 | Wendy             | Melvoin          |
 | Lisa              | Coleman          |

I could use "AS" to define an alias for each column, but that would be tedious. I want this to happen automatically.

  SELECT first_name AS person.first_name, last_name AS person.last_name FROM person;

The reason for my question is that I am using a database driver that does not provide the meta-data informing me the database column from where the result set got its data. I am trying to write generic code to handle the result set.

I would like to know how to do this in SQL generally, or at least in Postgres specifically.

SQLite had such a feature, though I see it is now inexplicably deprecated. SQLite has two pragma settings: full_column_names & short_column_names.

Answer 1

I know this question is a bit old, but perhaps someone will stumble over the answer and it will help them out.

The proper way to do what you're looking for is to create and use a View. Yes, it will be a bit tedious one-time to type out all those new column names as aliases, but if there are a lot of columns here's a trick you can use to leverage the PostgreSQL metadata to write out the text of the view:

select 'CREATE OR REPLACE VIEW people AS SELECT ' || 
(select string_agg(column_name || ' AS person_' || column_name, ', ')
from information_schema.columns
where table_name = 'person'
group by table_name) || 
' FROM person;';

running this yields:

?column?                                                 
------------------------------------------------------------------------------------------------------------- 
CREATE OR REPLACE VIEW people AS SELECT last_name AS person_last_name, first_name AS person_first_name FROM person; 

1 record(s) selected [Fetch MetaData: 0/ms] [Fetch Data: 0/ms]
[Executed: 4/21/12 2:05:21 PM EDT ] [Execution: 9/ms]

you can then copy and execute the results and voila:

select * from people;

 person_last_name     person_first_name    
 -------------------  -------------------- 
 Melvoin              Wendy                
 Coleman              Lisa                 

 2 record(s) selected [Fetch MetaData: 1/ms] [Fetch Data: 0/ms] 

Answer 2

To get the VIEW (Daryl's idea) in a single statement use a function or a DO command with EXECUTE:

DO
$do$
BEGIN

EXECUTE (
   SELECT format(
      'CREATE TEMP VIEW people AS SELECT %s FROM %I'
     , string_agg(format('%I AS %I', attname, attrelid::regclass || '.' || attname), ', ')
     , attrelid::regclass)
   FROM   pg_attribute
   WHERE  attrelid = 'person'::regclass  -- supply source table name once
   AND    attnum > 0
   AND    NOT attisdropped
   GROUP  BY attrelid
   );

END
$do$;

This immediately executes a command of the form:

CREATE OR REPLACE VIEW people AS
SELECT person_id AS "person.person_id"
     , first_name AS "person.first_name"
     , last_name AS "person.last_name"
FROM   person;

Would be less hassle to concatenate legal column names with '_' instead of '.'. But you need to be prepared for non-standard names that require double-quoting (and defend against possible SQL injection) anyway.

You can optionally provide a schema-qualified table name (myschema.person). The schema-name is prefixed in column names automatically if it is outside the current search_path.

For repeated use, you wrap this into a plpgsql function and make the table name a text parameter. All text-to-code conversion is sanitized here to prevent SQL injection. Example with more information here:

• Table name as a PostgreSQL function parameter

And you might use the new to_regclass() in Postgres 9.4+:

• How to check if a table exists in a given schema