Question

Why can’t I use a temporary column in the where clause?

For example, this query:

Select      product_brand,      (CASE WHEN COUNT(product_brand)>50 THEN 1 ELSE 0 END) AS brand_count  FROM      products  WHERE      1  GROUP BY      product_brand 

This brings up two columns, one called product_brand and one called brand_count. brand_count is created on the fly and is always 1 or 0 depending on whether or not there are 50 or products with that brand.

All this makes sense to me, except that I can’t select only if brand_count = 1 as in this query below:

Select      product_brand,     (CASE WHEN COUNT(product_brand)>50 THEN 1 ELSE 0 END) AS brand_count  FROM      products  WHERE     brand_count = 1  GROUP BY     product_brand 

which gives me this error:

#1054 - Unknown column 'brand_count' in 'where clause'  

Answer 1

Use HAVING instead:

Select     product_brand,     (CASE WHEN COUNT(product_brand)>50 THEN 1 ELSE 0 END) AS brand_count   FROM products   GROUP BY product_brand   HAVING brand_count = 1 

WHERE is evaluated before the GROUP BY. HAVING is evaluated after.