How to use a temp column in the where clause

问题:

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' 

回答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.

回答2:

Because in SQL the columns are first "selected" and then "projected".

回答3:

You have to use the full clause, so you will need:

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

This is the same for any calculated field in any SQL statement .

To simplify:

Select Max(Points) as Highest where Highest > 10

won't work, but:

Select Max(Points) as Highest where Max(Points) > 10

will. It's the same in your case.

回答4:

Because it has no idea what that column is until after it's done the processing.

If you want to access the column by that name you would have to use a subquery, otherwise you are going to have to qualify the column without the name you gave it, repeating your case statement.

回答5:

If I read your intent correctly, you can re-write this query to read:

Select 
 product_brand,
 COUNT(product_brand) AS brand_count 
FROM 
 products 
GROUP BY 
 product_brand
HAVING 
 COUNT(product_brand) > 50

This will give you all product_brands that have a count > 50 and will also show you the count for each.