BigQuery select alias using regex_extract_all in standard mode - sql

Question

I'm unable to reference a SELECT alias in BigQuery (standard mode).
Trying to do this query:
SELECT
REGEXP_EXTRACT_ALL(text,
r"(<div \w+>)") AS matches
FROM
regex.test
WHERE
matches IS NOT NULL
Here are steps to reproduce.
bq mk regex
bq mk -t regex.test id:integer,text:string
echo '{"id":1, "text":"<div a>"}' | bq insert regex.test
echo '{"id":2, "text":"<div b>"}' | bq insert regex.test
echo '{"id":3, "text":"<div>"}' | bq insert regex.test
bq query --use_legacy_sql=false "select REGEXP_EXTRACT_ALL(text, r\"(<div \w+>)\") AS matches FROM regex.test WHERE id IS NOT NULL"
+--------------+
| matches |
+--------------+
| [u'<div b>'] |
| [] |
| [u'<div a>'] |
+--------------+
When I try to reference the matches alias, I see an error:
bq query --use_legacy_sql=false "select REGEXP_EXTRACT_ALL(text, r\"(<div \w+>)\") AS matches FROM regex.test WHERE matches IS NOT NULL"
Error in query string: Error processing job 'myname': Unrecognized name:
matches
I am unable to reference the alias matches, and am unable to filter those results WHERE matches IS NOT NULL.
Does anyone know what I'm doing incorrectly here?
Thanks!

Answer 1

Even in BQ, you can't use a column alias in the where clause. Just use a subquery:
SELECT t.*
FROM (SELECT REGEXP_EXTRACT_ALL(text, r"(<div \w+>)") AS matches
FROM regex.test
) t
WHERE ARRAY_LENGTH(matches) > 0

Answer 2

Check out SELECT list aliases visibility
The reason why comparing with NULL does't work for REGEXP_EXTRACT_ALL is because
it returns array so checking with length is the way. Comparing with NULL still will work for REGEXP_EXTRACT
In addition, ideally you should be able use REGEX_MATCH to filter out records w/o matches, but looks like there is an issue with this function in standard mode