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N036-M4 Tier 3 · Intermediate · medium ecommerce · Brightlane

Return the result of combining `'42 Main Street'`, a SQL `NULL` unit, and `'Toronto'` with `', '` as the separator

Part of String Concatenation and Formatting in SQL

The problem

Brightlane's address formatter combines components with CONCAT_WS. Some records have no unit number on file, so the unit value is missing for those records.

Write a query to return the result of combining '42 Main Street', a SQL NULL unit, and 'Toronto' with ', ' as the separator.

Output:

  • A single row with one column, address, containing the concatenated string. The missing unit is omitted, with no doubled separator between the surrounding components.
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_WS(', ', '42 Main Street', NULL, 'Toronto') AS address

The shape

CONCAT_WS(', ', '42 Main Street', NULL, 'Toronto') skips the NULL unit argument entirely and joins the remaining values with one ', ' between them. The result is '42 Main Street, Toronto' with a single separator, not the doubled ', , ' a naive concatenation would produce.

Clause by clause

  • SELECT CONCAT_WS(', ', '42 Main Street', NULL, 'Toronto') AS address evaluates the CONCAT_WS call and labels the resulting column address. The first argument is the separator ', '; the remaining three arguments are the values to combine. CONCAT_WS walks the value list, ignores every argument that is NULL, and inserts the separator only between adjacent non-NULL values. The missing unit contributes nothing, and no separator is placed where it would have sat. There is no FROM because the arguments are literals.

Why this and not CONCAT('42 Main Street', ', ', NULL, ', ', 'Toronto')

CONCAT would treat the NULL unit as an empty string and still include the two separator literals on either side of it, producing '42 Main Street, , Toronto' with a doubled separator. CONCAT_WS understands that the separator is its job, not the caller's, and only inserts one between values that actually exist. Whenever the same delimiter is being placed between every pair of values and any value might be missing, CONCAT_WS is the right tool.

You practiced CONCAT_WS omitting a missing argument — the function leaves it out entirely without producing a doubled separator or a trailing delimiter.

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