operator does not exist: character varying = integer

You compared a text column to an integer column. PostgreSQL has no = operator for that pair of types and won’t convert one to the other for you. The full message comes with a hint:

ERROR:  operator does not exist: character varying = integer
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.

The hint suggests a cast. Usually the real fix is to compare the right columns.

Why it happens

c.email is VARCHAR(255) and o.customer_id is INTEGER:

SELECT o.id, c.first_name
FROM demo.orders o
JOIN demo.customers c ON c.email = o.customer_id;

PostgreSQL looks for an = that takes character varying on the left and integer on the right, and there isn’t one. The two types in the message are listed in the order they appear in the comparison, so character varying = integer means the text column is on the left.

Most other engines try to convert one side and carry on. PostgreSQL refuses, because the conversion is almost always hiding a mistake. An email can never equal a customer id, so the join could never match anything.

How to fix it

You’re comparing the wrong column. This is the common case, especially in joins. orders.customer_id points at customers.id, not at email:

SELECT o.id, c.first_name
FROM demo.orders o
JOIN demo.customers c ON c.id = o.customer_id;

When the error names a join condition, check the foreign key. The types on both sides of a correct join almost always match.

The types really differ, and the values are comparable. For example, an id stored as text in one table and as an integer in another. Cast one side explicitly:

SELECT id
FROM demo.customers
WHERE id::text = '1';

Cast the side whose conversion can’t fail. Every integer converts to text, but c.email::integer fails with invalid input syntax for type integer as soon as it reaches 'alice@example.com'. Casting a column also stops PostgreSQL from using an index on it, so if you do this often, fix the column type instead.

Why a quoted number works but a typed one doesn’t

This runs fine, even though '1' looks like text:

SELECT id FROM demo.orders WHERE customer_id = '1';

A quoted literal has no type until PostgreSQL decides what it should be, and next to an integer column it becomes an integer. A value that already has a type is different: customer_id = 1::text, or a parameter your driver sends as text, gives operator does not exist: integer = text. If the error appears only when the query runs from application code, check what type the driver sends for the parameter.