Asked  6 Months ago    Answers:  5   Viewed   36 times

I need to use an alias in the WHERE clause, but It keeps telling me that its an unknown column. Is there any way to get around this issue? I need to select records that have a rating higher than x. Rating is calculated as the following alias:

sum(reviews.rev_rating)/count(reviews.rev_id) as avg_rating

 Answers

90

You could use a HAVING clause, which can see the aliases, e.g.

 HAVING avg_rating>5

but in a where clause you'll need to repeat your expression, e.g.

 WHERE (sum(reviews.rev_rating)/count(reviews.rev_id))>5

BUT! Not all expressions will be allowed - using an aggregating function like SUM will not work, in which case you'll need to use a HAVING clause.

From the MySQL Manual:

It is not allowable to refer to a column alias in a WHERE clause, because the column value might not yet be determined when the WHERE clause is executed. See Section B.1.5.4, “Problems with Column Aliases”.

Tuesday, June 1, 2021
 
maniclorn
answered 6 Months ago
69

Instead of using a subquery, you may get the values from a left join and count the distinct not null values.

SELECT meta_name, meta_value, COUNT(DISTINCT pid) count
  FROM meta m JOIN item_meta im
    ON im.field_id = m.id
LEFT JOIN (
    SELECT i.id pid
  FROM item_meta im JOIN items i
    ON im.item_id = i.id JOIN meta m
    ON im.field_id = m.id
 GROUP BY i.id
HAVING MAX(meta_name = 'Make' AND meta_value = 'BMW') = 1
   AND MAX(meta_name = 'Car Type' AND meta_value = 'Coupe') = 1)
LJ ON im.item_id = LJ.pid
 GROUP BY meta_name, meta_value;
Saturday, May 29, 2021
 
Parfait
answered 7 Months ago
42

This is not possible directly, because chronologically, WHERE happens before SELECT, which always is the last step in the execution chain.

You can do a sub-select and filter on it:

SELECT * FROM
(
  SELECT A.identifier
    , A.name
    , TO_NUMBER(DECODE( A.month_no
      , 1, 200803 
      , 2, 200804 
      , 3, 200805 
      , 4, 200806 
      , 5, 200807 
      , 6, 200808 
      , 7, 200809 
      , 8, 200810 
      , 9, 200811 
      , 10, 200812 
      , 11, 200701 
      , 12, 200702
      , NULL)) as MONTH_NO
    , TO_NUMBER(TO_CHAR(B.last_update_date, 'YYYYMM')) as UPD_DATE
  FROM table_a A
    , table_b B
  WHERE A.identifier = B.identifier
) AS inner_table
WHERE 
  MONTH_NO > UPD_DATE

Interesting bit of info moved up from the comments:

There should be no performance hit. Oracle does not need to materialize inner queries before applying outer conditions -- Oracle will consider transforming this query internally and push the predicate down into the inner query and will do so if it is cost effective. – Justin Cave

Saturday, June 5, 2021
 
mistero
answered 6 Months ago
29

There are two problems with your SQL:

  1. (THis is not the question, but should be considered) by using WHERE over the UNION instead of the tables, you create a performance nightmare: MySQL will create a temporary table containing the UNION, then query it over the WHERE. Using a calculation on a field (LOWER(requester_name)) makes this even worse.

  2. The reason you get two rows is, that UNION DISTINCT will only suppress real duplicates, so the tuple (someuser,peter) and the tuple (someotheruser, peter) will result in duplication.

Edit

To make (someuser, peter) a duplicate of (peter, someuser) you could use:

SELECT
  IF(requester_name='peter', receiver_name, requester_name) AS otheruser
FROM
  ...
UNION
SELECT
  IF(requester_name='peter', receiver_name, requester_name) AS otheruser
FROM
  ...

So you only select someuser which you already know : peter

Tuesday, September 21, 2021
 
g.pickardou
answered 2 Months ago
52

I can't use my alias in the where clause.

1. Is this approach possible?

Sure, put it in a subquery.

SELECT *
FROM
(
SELECT stories.id, [stories.title] & " " & [stories.author] & " " & [stories.publisher] & " " & [stories.memo] AS allMyText
FROM stories
) AS SUBQ
WHERE ((([allMyText]) Like "*apples*" And ([allMyText]) Not Like "*applesauce*"));

2. Wouldn't the alternative mean that I'd be performing multiple string concatenations on every row iteration?

Yes that is right, the alternative is to repeat the expression. I won't bore you with the code for this alternative.

For your particular query, you can also use this

SELECT stories.id, [stories.title] & " " & [stories.author] & " " & [stories.publisher] & " " & [stories.memo] AS allMyText
FROM stories
WHERE ([stories.title] Like "*apples*" OR [stories.author] Like "*apples*" 
  OR [stories.publisher] Like "*apples*" OR [stories.memo] Like "*apples*")
AND NOT ([stories.title] Like "*applesauce*" OR [stories.author] Like "*applesauce*"
  OR [stories.publisher] Like "*applesauce*" OR [stories.memo] Like "*applesauce*")
Thursday, November 18, 2021
 
Peteris
answered 2 Weeks ago
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