Question

I have about 5 tables with common field “userid” I want to count distinct userids across all these tables. Certain user ids may occur in about 2 of these tables. But I want to count the UNION of distinct user ids in the 5 tables together. I am able to count distinct userids in one table with the ff code. I want to be able count from all tables.

//just count the nuber of distict userids in table1
    $query2 = ("SELECT COUNT(DISTINCT userid) FROM table1"); 
    $result2 = mysql_query($query2) or die(mysql_error()); 
    $row2 = mysql_fetch_row($result2); 
    echo "Number of users in table 1  = ".$row2[0] ."<br />";


**Table 1**
Id    userid    name   adresss

**Table 2**
Id  Title   Sex   userid

**Table 3**
Id   userid   amount

**Table 4**
Id  price  promotion  userid   productid

**Table  5**
Id  userid   category   tax   weight

Answer 1

Use a UNION

A standard UNION, in contrast with a UNION ALL, will remove duplicate values from the resultset. Your statement would look like

SELECT   COUNT(*)
FROM     (
           SELECT UserID FROM Table1
           UNION SELECT UserID FROM Table2
           UNION SELECT UserID FROM Table3
           UNION SELECT UserID FROM Table4
           UNION SELECT UserID FROM Table5
         ) AS UserIDs

From w3schools

the UNION operator selects only distinct values by default. To allow
duplicate values, use UNION ALL.