Level 2 · Pick only the columns you need

Most real questions only need a few columns. You list them after SELECT, separated by commas:

SELECT name, category, unit_price
FROM products;

The result has exactly those columns, in exactly the order you wrote them. If you want price first, write unit_price first. The order of columns in the result has nothing to do with the order in the table.

Why this matters on the job

An explicit column list is a small contract. It tells whoever reads your query, including you in three months, which data the report depends on. It also keeps the result stable when the table changes: if an engineer adds a supplier_notes column to products next week, this query still returns the same three columns.

In column-oriented warehouses (BigQuery, Snowflake, Redshift, and DuckDB itself), selecting fewer columns is also faster, because each column is stored separately and unused columns are never read.

Formatting

SQL ignores extra whitespace, so these two queries are identical:

SELECT name, category, unit_price FROM products;

SELECT
  name,
  category,
  unit_price
FROM products;

Teams usually put one column per line once a query has more than three or four columns. It makes diffs in code review cleaner and makes it easy to comment out a single column with --, which starts a comment that runs to the end of the line:

SELECT
  name,
  -- category,
  unit_price
FROM products;

Common mistakes

• Trailing comma. SELECT name, category, FROM products is an error in most databases. DuckDB and BigQuery happen to tolerate a trailing comma in the select list, Postgres and MySQL do not. Do not get used to it.
• Missing comma. SELECT name category FROM products is not an error. It returns one column called category that actually contains name, because the second word is read as a new name for the first column. This silent bug is common, and it is a nice segue into aliases, which are next.
• Quoting column names with single quotes. SELECT 'name' FROM products returns the literal text “name” on every row. Single quotes are for text values; column names are written bare.