How to Efficiently Combine Columns in PostgreSQL Using SQL?

Combine Columns with an SQL Statement

When combining columns into a new column in PostgreSQL, there are several approaches to consider.

Concatenation Operators

The traditional concatenation operators, || and concat(), offer straightforward options. || evaluates to NULL if either operand is NULL, while concat() returns an empty string in such cases.

COALESCE

To handle NULL values, COALESCE can be used to replace NULLs with empty strings or other default values before concatenation.

CASE Expressions

CASE expressions provide flexibility in handling NULL values. By evaluating each column individually and specifying the desired output based on the nullity, you can control the behavior of the concatenation.

concat_ws

concat_ws() is particularly useful for concatenating multiple columns with custom separators.

Examples

1. Basic concatenation:

SELECT col_a || col_b;

2. Handling NULL values with COALESCE:

SELECT COALESCE(col_a, '') || COALESCE(col_b, '');

3. Concatenating multiple columns with separators:

SELECT concat_ws(' - ', col_a, col_b, col_c);

4. Handling NULL values in a complex query:

SELECT CASE
  WHEN (col_a, col_b) IS NULL THEN NULL
  ELSE concat(col_a, col_b)
END;

Considerations

• NULL values: Choose the appropriate concatenation method based on how you want to handle NULLs.
• Performance: If nullity is common, avoiding COALESCE or CASE expressions can improve performance.
• Data types: Ensure that the data types of the columns being concatenated are compatible.

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