SQL query to join two tables with no repeated values? - mysql

Question

Table 1
ID | NAME | WARD_ID|
1 A 1
2 B 1
3 C 2
4 D 2
Table 2
ID | MONTH1 | MONTH2 | WARD_ID|
1 9 10 1
2 6 11 1
3 5 12 2
4 13 14 2
I want to join this two table and produce the following output:
ID | NAME | MONTH1 | MONTH2 | WARD_ID|
1 A 9 10 1
2 B 6 11 1
3 C 5 12 2
4 D 13 14 2
In the ON condition of the query I have to keep WARD_ID equal for both the tables. I could not able to figure out the solution. Anyone have any experience with a query like this?

Answer 1

I think you want something like this:
select t1.*, t2.*
from (select t1.*,
(#rn1 := if(#w1 = ward_id, #rn1 + 1,
if#w1 := ward_id, 1, 1)
)
) as rn
from (select t1.* from table1 t1 order by ward_id, id ) t1 cross join
(select #w1 := -1, #rn1 := -1) params
) t1 join
(select t2.*,
(#rn2 := if(#w2 = ward_id, #rn2 + 1,
if#w2 := ward_id, 1, 1)
)
) as rn
from (select t2.* from table2 t2 order by ward_id, id ) t2 cross join
(select #w2 := -1, #rn1 := -1) params
) t1
on t2.ward_id = t1.ward_id and t2.rn = t1.rn;
The subqueries enumerate the rows in each table. The join then uses the enumeration.
This is much simpler in MySQL 8.0, using row_number().

Answer 2

I'm assuming here that ID is intended to be the same from both tables. If so, I think you can do a multi-condition join:
select * from table1 t1
inner join table2 t2
on t1.ID=t2.ID and t1.WARD_ID=t2.WARD_ID

Answer 3

You can do something like:
SET #rn:=0;
SET #rn2:=0;
SELECT *
FROM (
SELECT #rn:=#rn+1 AS rn1, t1.ID, t1.NAME, t1.WARD_ID
FROM t1
GROUP BY t1.WARD_ID, t1.NAME
ORDER BY t1.WARD_ID, t1.NAME
) s1
INNER JOIN (
SELECT #rn2:=#rn2+1 AS rn2, t2.ID, t2.MONTH1, t2.MONTH2, t2.WARD_ID
FROM t2
GROUP BY t2.WARD_ID, t2.MONTH1,t2.MONTH2
ORDER BY t2.WARD_ID, t2.MONTH1,t2.MONTH2
) s2 ON s1.WARD_ID = s2.WARD_ID
AND s1.rn1 = s2.rn2
But it really doesn't reliably sort the tables to join the same rows every time. I still think there isn't a reliable/repeatable way to join the two tables the same every time.
============================================================
http://sqlfiddle.com/#!9/aa2db0/1 <<<< If ID can be used to reliably sort the two tables, you can use it in the ORDER BYs. I've added it in this Fiddle, and included rows in the setup that would fall before the existing records and potentially change the sorting. This also includes more records in Table 2 than there are in Table 1, so would possibly result in duplicated rows. These new rows are ignored since they can't be matched between tables.