SQL Query - Set Date Variable by Table/Column Reference
If I understand your question correctly, there are two ways that I can think of:

mysql&gt; SET @date = (SELECT source_col FROM source_table WHERE whatever_condition); 
Query OK, 1 row affected (0.03 sec)


or

mysql&gt; SELECT source_col FROM source_table WHERE whatever_condition INTO @date;
Query OK, 1 row affected (0.03 sec)


These two are functionally equivalent when only one value is involved, but the second form can also be expanded to handle multiple values.

mysql&gt; SELECT col1, col2 FROM source_tbl WHERE whatever_condition INTO @var1, @var2;


But your question involved using the resulting @date in your query.

mysql&gt; SELECT * 
         FROM target_table 
        WHERE target_column = @date;


If that's the intended purpose, you can eliminate the variable entirely and use the first example, above, as a subquery inside the final query.  

mysql&gt; SELECT * 
         FROM target_table 
        WHERE target_column = (SELECT source_col FROM source_table 
                                WHERE whatever_condition);


http://dev.mysql.com/doc/refman/5.5/en/comparisons-using-subqueries.html