UNION SQL Issue [duplicate]

Question

This question already has answers here:

mysql_fetch_array()/mysql_fetch_assoc()/mysql_fetch_row()/mysql_num_rows etc… expects parameter 1 to be resource or result (32 answers)

Closed 6 years ago.

I recently noticed the UNION function and saw on W3Schools that you can unite two table, but I want to unite three... I have made up a piece of code, however it doesn't work because I get an error. Is it possible to unite three tables and how can I achieve this? Thanks!

ERROR:

Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean given in C:\wamp\www\Legendary\new\search.php on line 54

CODE:

            // Fill up array with names
            $sql2 = mysql_query("SELECT * FROM members UNION ALL SELECT * FROM clans UNION ALL SELECT * FROM tournaments"); 
            while($row=mysql_fetch_array($sql2)) {
                $a[]=$row['name'];
                $b[]=$row['id'];
            }

Answer 1

You need to be returning the same number of columns and data types from each table and in the same order (because mySQL does not support the SQL standard's UNION CORRESPONDING syntax) . If the tables do not match exactly, but do have some similar columns that you would like to UNION, then specify only those columns in the SELECT clause for each part of the UNION.

Even if the table schemas match, it is good practice to specify the column names; otherwise adding a column to one of the tables in the future would break the query.

For example, if all tables had a Name column, you could do:

select Name from members
UNION ALL
select Name from clans
UNION ALL
select Name from tournaments

Answer 2

UNION is a SQL command to STACK the results on top of other results, which is why you need the same number of columns. If you are looking to see which clans and tournaments certain members have participated in, then you need to JOIN them.