WHERE

Filters the rows of the FROM source and its joins before any grouping. A row is kept only when the condition is true; false and NULL both drop it.

Syntax

SELECT ...
FROM <source> [ <alias> ]
  [ <join> ... ]
WHERE <condition>
[ GROUP BY ... ]
...

Parameter	Description
`<condition>`	A boolean expression: comparisons, `AND`, `OR`, `NOT`, `IN`, `EXISTS`, `BETWEEN`, `LIKE`, `IS NULL`, a boolean column, or a function that returns a boolean such as `regexp_like`. See Operators.

A comparison with NULL is unknown, so o.status <> 'closed' does not keep rows whose status is NULL. Add OR o.status IS NULL to keep them.
In a text, date or timestamp column an empty string is NULL.
A plain column compared with a literal (o.status = 'open', o.amount > 100, o.region IN ('AU', 'NZ')) is pushed into the read of the table, so only matching rows are read. Expressions are pushed too when the data store can evaluate them the same way; otherwise they run after the rows are read.
An unquoted number compared with a text column compares as a number: t.code = 7 matches '7' and '07'.
WHERE runs before GROUP BY. To filter on an aggregate, use HAVING. To filter on a window function, compute it in a derived table and filter outside.
A subquery can appear as IN (SELECT ...), EXISTS (SELECT ...) or <column> <op> (SELECT ...). See Subqueries.

SELECT o.key, o.amount
FROM Order o
WHERE o.status IN ('open', 'held')
  AND o.amount >= 1000
  AND (o.title ILIKE '%refund%' OR o.closed_at IS NULL)

key	amount
PO-1001	1200
PO-1017	4500

A condition over an expression:

SELECT t.path FROM tables.drive.file t
WHERE lower(split_part(t.path, '/', 1)) = 'finance' AND length(t.name) > 20

Rows whose value is not in another table:

SELECT c.key FROM Customer c
WHERE NOT EXISTS (SELECT r.to_key FROM placed r WHERE r.from_key = c.key)

Refused	Write instead
An aggregate in `WHERE`	Filter on it in `HAVING`
A window function in `WHERE`	Compute it in a derived table and filter outside

-- Refused by the engine
SELECT o.customer FROM Order o WHERE COUNT(*) > 5 GROUP BY o.customer

-- Refused by the engine
SELECT o.key FROM Order o WHERE ROW_NUMBER() OVER (ORDER BY o.amount DESC) <= 3