Invalid Identifier when using join - sql

Question

Im trying to join two tables with a left join but, the next error comes out:
error line 7 at position 11 invalid identifier 'VAR01'
This is my query
SELECT
CU.X||CU.Y||CU.Z AS VAR01,
RE.VAR02,
*
FROM TABLE01 CU
LEFT JOIN TABLE02 RE
ON VAR01 = RE.VAR02
LIMIT 10
I know that the Invalid identifiers implies to me something is named incorrectly, but all the variables that Im using are correct

Answer 1

I could not reproduce this case:
CREATE OR REPLACE TABLE TABLE01(X TEXT, Y TEXT, Z TEXT)
AS SELECT 'a', 'b', 'c'
UNION SELECT 'x', 'y', 'z'
UNION SELECT 'i', 'j', 'k';
CREATE OR REPLACE TABLE TABLE02(VAR02 TEXT)
AS SELECT 'abc'
UNION SELECT 'ijk';
SELECT
CU.X||CU.Y||CU.Z AS VAR01,
RE.VAR02,
*
FROM TABLE01 CU
LEFT JOIN TABLE02 RE
ON VAR01 = RE.VAR02
LIMIT 10;
Query profile:
EXPLAIN USING TABULAR
SELECT
CU.X||CU.Y||CU.Z AS VAR01,
RE.VAR02,
*
FROM TABLE01 CU
LEFT JOIN TABLE02 RE
ON VAR01 = RE.VAR02
LIMIT 10;

Answer 2

Looks like you were missing the alias from your first table before the column name. Your join was trying to join on
ON VAR01 = RE.VAR02
You need the table alias before the VAR01, see below:
SELECT
CU.X||CU.Y||CU.Z AS VAR01,
RE.VAR02,
*
FROM TABLE01 CU
LEFT JOIN TABLE02 RE
ON CU.VAR01 = RE.VAR02
LIMIT 10

Answer 3

This is because the ON clause is evaluated before the SELECT clause, so the alias VAR01 does not yet exist at the time the ON clause is being evaluated.
You can do it either way :
1) Repeat the expression in the ON clause :
SELECT
CU.X||CU.Y||CU.Z AS VAR01,
RE.VAR02,
*
FROM TABLE01 CU
LEFT JOIN TABLE02 RE
ON CU.X||CU.Y||CU.Z = RE.VAR02
LIMIT 10;
2) Using Subquery:
SELECT *
FROM (
SELECT
CU.X||CU.Y||CU.Z AS VAR01,
RE.VAR02,
*
FROM TABLE01 CU
LEFT JOIN TABLE02 RE
ON CU.X||CU.Y||CU.Z = RE.VAR02
LIMIT 10
) subquery;