使用PHP和MySQL从多个表的多个列中选择DISTINCT

Question

Using PHP and MySQL I'm trying to get all the distinct values out the number and type columns from all 4 of the tables listed below:

table_1

ID|t1_number|t1_type
1|1|new
2|1|old
3|2|new
4|3|new
5|1|old

table_2

ID|t2_number|t2_type
1|1|future
2|1|new
3|3|past
4|3|new
5|1|new

table_3

ID|t3_number|t3_type
1|1|past
2|1|new
3|1|new
4|1|new
5|1|old

table_4

ID|t4_number|t4_type
1|1|new
2|4|new
3|3|old
4|2|new
5|1|new

The values I want from the above tables would be:

numbers: 1,2,3,4

types: new,old,future,past

Here is what I have so far; but I'm not sure if the SQL is correct or how to format the while loop to get the values out.

$sql = "SELECT DISTINCT table_1.t1_number, table_2.t2_number, table_3.t3_number, table_4.t4_number, table_1.t1_type, table_2.t2_type, table_3.t3_type, table_4.t4_type
FROM table_1 
JOIN table_2
JOIN table_3
JOIN table_4";

$result = @mysql_query($sql, $con) or die(mysql_error());

while($row = mysql_fetch_array($result)) {

    $numbers= $row[?????????];

}

Answer 1

Doing this in a single query risks duplicating a value in the output for either the number or type column, in the case that one list has more values than the other. To get a distinct list of values for either column, this needs to be separate queries:

Number

SELECT t1_number AS num
  FROM TABLE_1
UNION
SELECT t2_number
  FROM TABLE_2
UNION
SELECT t3_number
  FROM TABLE_3
UNION
SELECT t4_number
  FROM TABLE_4

Type

SELECT t1_type AS type
  FROM TABLE_1
UNION
SELECT t2_type
  FROM TABLE_2
UNION
SELECT t3_type
  FROM TABLE_3
UNION
SELECT t4_type
  FROM TABLE_4

There's no value to running DISTINCT in this UNION query, because duplicates will be removed, and this deals with only one column.

Answer 2

It's not completely clear what you're asking for. Do you want the four distinct numbers, and then the four distinct types?

SELECT t1_number FROM table_1
UNION SELECT t2_number FROM table_2
UNION SELECT t3_number FROM table_3
UNION SELECT t4_number FROM table_4;

SELECT t1_type FROM table_1
UNION SELECT t2_type FROM table_2
UNION SELECT t3_type FROM table_3
UNION SELECT t4_type FROM table_4;

Or do you want each distinct pair of number & type?

SELECT t1_number, t1_type FROM table_1
UNION SELECT t2_number, t2_type FROM table_2
UNION SELECT t3_number, t3_type FROM table_3
UNION SELECT t4_number, t4_type FROM table_4;

The neat thing about UNION is that it implicitly reduces the result to distinct rows. You can preserve duplicates if you use UNION ALL.

Answer 3

With SQL:

SELECT DISTINCT t1_number AS num FROM t1
    WHERE num NOT IN (SELECT t2_number FROM t2)
    AND num NOT IN (SELECT t3_number FROM t3)
    AND num NOT IN (SELECT t4_number FROM t4)

The same idea will work for the types.

Answer 4

I think this is what you want:

select distinct number_value, type_value from (
  select t1_number as number_value, t1_type as type_value from table_1
union
  select t2_number as number_value, t2_type as type_value from table_2
union
  select t3_number as number_value, t3_type as type_value from table_3
union
  select t4_number as number_value, t4_type as type_value from table_4
)