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N036-M2 Tier 3 · Intermediate · medium ecommerce · Brightlane

Return the result of using `CONCAT` to combine `'Sarah'` with a SQL `NULL`

Part of String Concatenation and Formatting in SQL

The problem

Brightlane's display system builds customer labels by combining a first name and a middle name. Some customers have no middle name on record.

Write a query to return the result of using CONCAT to combine 'Sarah' with a SQL NULL.

Output:

  • A single row with one column, display_name, containing the concatenated string. CONCAT treats a missing input as an empty string, so the result is the present value alone.
Schema · ecommerce 5 tables
categories
id integer
name text
parent_id? integer
products
id integer
name text
category_id integer
price numeric
stock_qty integer
attributes? jsonb
order_items
id integer
order_id integer
product_id integer
quantity integer
unit_price numeric
customers
id integer
name text
email text
city? text
country text
created_at timestamptz
is_active boolean
orders
id integer
customer_id integer
ordered_at timestamptz
status text
total_amount numeric
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Solution query
SELECT
  CONCAT('Sarah', NULL) AS display_name

The shape

CONCAT('Sarah', NULL) returns 'Sarah' because CONCAT treats a NULL argument as an empty string and concatenates around it. The missing middle name is simply omitted from the output rather than poisoning it.

Clause by clause

  • SELECT CONCAT('Sarah', NULL) AS display_name evaluates the CONCAT call and labels the resulting column display_name. CONCAT walks its argument list and concatenates each non-NULL value in order, substituting an empty string for any NULL it encounters. The first argument 'Sarah' is included; the second argument NULL contributes nothing; the combined result is the first value alone. There is no FROM because both arguments are literals.

Why this and not 'Sarah' || NULL

'Sarah' || NULL would return NULL, not 'Sarah'. The || operator propagates NULL through the result the moment either side is NULL, so a missing middle name would erase the whole label. CONCAT is the right tool here precisely because the display system needs the present value to come through whenever any component happens to be missing. The choice between || and CONCAT is a NULL-semantics decision: || when missing should invalidate the result, CONCAT when missing should be skipped.

You practiced CONCAT(...) treating a missing input as an empty string — the right tool when a missing argument should be omitted from the concatenated output rather than poisoning it.

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