[sql] Combine two columns and add into one new column

In PostgreSQL, I want to use an SQL statement to combine two columns and create a new column from them.

I'm thinking about using concat(...), but is there a better way?
What's the best way to do this?

This question is related to sql postgresql null concatenation

The answer is

Did you check the string concatenation function? Something like:

update table_c set column_a = column_b || column_c 

should work. More here

You don't need to store the column to reference it that way. Try this:

To set up:

  (zipcode text NOT NULL, city text NOT NULL, state text NOT NULL);
INSERT INTO tbl VALUES ('10954', 'Nanuet', 'NY');

We can see we have "the right stuff":

\pset border 2
| zipcode |  city  | state |
| 10954   | Nanuet | NY    |

Now add a function with the desired "column name" which takes the record type of the table as its only parameter:

CREATE FUNCTION combined(rec tbl)
  RETURNS text
AS $$
  SELECT $1.zipcode || ' - ' || $1.city || ', ' || $1.state;

This creates a function which can be used as if it were a column of the table, as long as the table name or alias is specified, like this:

SELECT *, tbl.combined FROM tbl;

Which displays like this:

| zipcode |  city  | state |      combined      |
| 10954   | Nanuet | NY    | 10954 - Nanuet, NY |

This works because PostgreSQL checks first for an actual column, but if one is not found, and the identifier is qualified with a relation name or alias, it looks for a function like the above, and runs it with the row as its argument, returning the result as if it were a column. You can even index on such a "generated column" if you want to do so.

Because you're not using extra space in each row for the duplicated data, or firing triggers on all inserts and updates, this can often be faster than the alternatives.

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