column "x" does not exist

PostgreSQL looked for a column with that name in the tables listed in FROM and didn’t find one. If the name is obviously spelled right, the cause is almost always one of three things: it’s an alias from SELECT, it’s a string in double quotes, or it’s an identifier whose case doesn’t match.

Why it happens

stock_value isn’t a column of demo.products. It’s a name the SELECT list gives to an expression:

SELECT name, price * stock_quantity AS stock_value
FROM demo.products
WHERE stock_value > 5000;

The clauses don’t run in the order they’re written. The database works out FROM first, then filters with WHERE, then groups, and only then evaluates SELECT. When WHERE runs, stock_value hasn’t been computed yet, so there’s nothing by that name to compare against.

ORDER BY runs after SELECT, which is why the same alias works there:

SELECT name, price * stock_quantity AS stock_value
FROM demo.products
ORDER BY stock_value DESC;

How to fix it

Repeat the expression in WHERE. It’s the shortest fix:

SELECT name, price * stock_quantity AS stock_value
FROM demo.products
WHERE price * stock_quantity > 5000;

Seven products have more than 5,000 worth of stock.

Compute it once in a subquery when the expression is long, or used in several places. The outer query sees the alias as an ordinary column:

SELECT *
FROM (
  SELECT name, price * stock_quantity AS stock_value
  FROM demo.products
) p
WHERE stock_value > 5000;

A CTE (WITH p AS (...) SELECT ... FROM p WHERE ...) does the same job and reads top to bottom.

If the alias is an aggregate, such as COUNT(*) AS orders, the filter belongs in HAVING, not WHERE. See aggregate functions are not allowed in WHERE.

Double quotes are for names, not strings

In standard SQL, and in PostgreSQL, double quotes mark an identifier. Single quotes mark a string. This query asks for rows where country equals a column called US:

SELECT first_name
FROM demo.customers
WHERE country = "US";          -- column "US" does not exist

Use single quotes for the value:

SELECT first_name
FROM demo.customers
WHERE country = 'US';

Alice, Bob and Iris come back. MySQL treats double quotes as strings by default, so queries written for MySQL hit this the moment they move to PostgreSQL.

Quoted identifiers are case-sensitive

PostgreSQL folds unquoted names to lower case. SELECT First_Name works, because it becomes first_name. Put the name in double quotes and the folding stops:

SELECT "First_Name" FROM demo.customers;
-- ERROR:  column "First_Name" does not exist
-- HINT:  Perhaps you meant to reference the column "customers.first_name".

The reverse catches people too. A table created by a tool that quoted mixed-case names, such as "createdAt", has to be quoted with that exact case every time. Unquoted, createdAt folds to createdat, which doesn’t exist.

When the message comes with a HINT, read it. PostgreSQL suggests a close match for typos and case mismatches, which is often the whole answer.