How to treat NULL values as 0 from MySQL

Question

I have two tables joined by user_id . In the query below I'm comparing act_val in table_two to make sure it's in between or equal to min_val and max_val from table_one. But sometimes a user_id in table_one isn't found in table_two. In those cases, I want the act_val to be treated as if it were 0 and qualify where table_one's min_val is lower. Is that possible? How so?

SELECT tb1.id FROM table_one tb1 
INNER JOIN table_two tb2 WHERE 
   tb1.min_val <= tb2.act_val AND tb1.min_val >= tb2.act_val 
   AND tb2.user_id = tb1.user_id 
GROUP BY ad.id;

Answer 1

You need the user_id link in the LEFT JOIN condition (ON clause), and the rest in the WHERE clause (filter). tb2.act_val is treated as 0 when it is missing, using IFNULL . I also fixed up your 2nd condition, it should be tb1. max _val >= not tb1. min _val >=

     SELECT tb1.id
        FROM table_one tb1 
   LEFT JOIN table_two tb2 ON tb2.user_id = tb1.user_id
       WHERE tb1.min_val <= IFNULL(tb2.act_val,0)
         AND tb1.max_val >= IFNULL(tb2.act_val,0)
    GROUP BY tb1.id;

LEFT JOIN :

If there is no matching row for the right table in the ON or USING part in a LEFT JOIN, a row with all columns set to NULL is used for the right table. You can use this fact to find rows in a table that have no counterpart in another table:

Answer 2

Use an OUTER join (LEFT in the following example) to get the all the TABLE1 records that may or may not have a matching user_id value in TABLE2. Then construct the WHERE clause to filter down the record set to what you want:

   SELECT tb1.id
     FROM table_one tb1 
LEFT JOIN table_two tb2 ON tb2.user_id = tb1.user_id 
    WHERE tb2.act_val BETWEEN tb1.min_val AND tb1.max_val
       OR tb1.min_val < 0
 GROUP BY tb1.id

Answer 3

Use the IS_NULL or IS_NOT_NULL operators. Please see http://dev.mysql.com/doc/refman/5.0/en/working-with-null.html

Answer 4

您可以使用IFNULL(tbl2.act_val, 0)

Answer 5

1. Replace inner join to left join
2. Use coalesce(tb2.act_val, 0) instead of tb2.act_val

PS Also - your query looks incorrect because you refers to undefined table ad.

Answer 6

您可以使用COALESCE函数：

WHERE COALESCE(tb1.min_val, 0) <= tb2.act_val COALESCE(tb1.min_val, 0) >= tb2.act_val