MySQL Error: SELECT list is not in GROUP BY clause

Question

I have a problem with my query and mysql throws the following error:

#1055 - Expression #66 of SELECT list is not in GROUP BY clause and 
contains nonaggregated column 's.status' which is not functionally 
dependent on columns in GROUP BY clause; this is incompatible with 
sql_mode=only_full_group_by

Query is:

select   p.*,
pd.*,
m.*,
IF(s.status, s.specials_new_products_price, null) as specials_new_products_price,
IF(s.status, s.specials_new_products_price, p.products_price) as final_price
FROM products p 
LEFT JOIN specials s ON p.products_id = s.products_id  
LEFT JOIN manufacturers m using(manufacturers_id) , 
          products_description pd,
          categories c,
          products_to_categories p2c
WHERE p.products_view = 1  
AND p.products_status = 1
AND p.products_archive = 0
AND c.virtual_categories = 0
AND p.products_id = pd.products_id
AND p.products_id = p2c.products_id
AND p2c.categories_id = c.categories_id
AND pd.language_id = 1

GROUP BY p.products_id;

Answer 1

When you use GROUP BY, you can use expressions in your select-list only if they have a single value per group. Otherwise you get ambiguous query results.

In your case, MySQL believes that s.status might have multiple values per group. For example, you're grouping by p.products_id but s.status is a column in another table specials, perhaps in a one-to-many relationship with table products. So there might be multiple rows in specials with the same products_id, but different values for status. If that's the case, which value for status should the query use? It's ambiguous.

In your data, you might happen to limit the rows such that you only have one row in specials for each row in products. But MySQL can't make that assumption.

MySQL 5.6 and earlier let you write such ambiguous queries, trusting that you know what you're doing. But MySQL 5.7 enables more strict enforcement by default (this can be made less strict to behave like earlier versions).

The fix is to follow this rule: Every column in your select-list must fall into one of three cases:

• The column is inside an aggregate function like COUNT(), SUM(), MIN, MAX(), AVERAGE(), or GROUP_CONCAT().
• The column is one of the column(s) named in the GROUP BY clause.
• The column is functionally dependent on the column(s) named in the GROUP BY clause.

For more explanation read this excellent blog: Debunking GROUP BY myths

---

Re your comment, I can only make a guess because you have not posted your table definitions.

I'm guessing that products_description and manufacturers are functionally dependent on products, so it's okay to list them as is in the select-list. But this assumption may not be correct, I don't know your schema.

Anyway, the error about s.status should be resolved by using an aggregate function. I'm using MAX() as an example.

SELECT p.*,
pd.*,
m.*,
MAX(IF(s.status, s.specials_new_products_price, NULL)) 
  AS specials_new_products_price,
MAX(IF(s.status, s.specials_new_products_price, p.products_price)) 
  AS final_price
FROM products p 
LEFT OUTER JOIN specials s ON p.products_id = s.products_id  
INNER JOIN manufacturers m ON p.manufacturers_id = m.manufacturers_id
INNER JOIN products_description pd ON p.products_id = pd.products_id
INNER JOIN products_to_categories p2c ON p.products_id = p2c.products_id
INNER JOIN categories c ON p2c.categories_id = c.categories_id
WHERE p.products_view = 1  
AND p.products_status = 1
AND p.products_archive = 0
AND c.virtual_categories = 0
AND pd.language_id = 1
GROUP BY p.products_id;

I also rewrote your joins in the proper way. Comma-style joins should be avoided.