How to use union for handling date from two different tables - mysql

Question

I have two tables. First table stk
ITEMID | STOCK
--------------
Test1 | 10
Test2 | 15
Test3 | 12
and second table qty
ITEMID | DOCDATE | QTY
--------------------------
Test1 | 2/28/2017 | 5
Test2 | 2/28/2017 | 8
Test3 | 2/28/2017 | 6
I used this query
select itemid,stock,docdate,qty
from (
select itemid,stock,null docdate,0 qty from stk
union
select itemid,0 stock,docdate,qty from qty
)
group by itemid,stock,docdate,qty
order by 1
Output I get:
ITEMID | STOCK | DOCDATE | QTY
------------------------------
Test1 | 0 |2/28/2017 | 5
Test1 | 10 | | 0
Test2 | 0 |2/28/2017 | 8
Test2 | 15 | | 0
Test3 | 0 |2/28/2017 | 6
Test3 | 12 | | 0
but I want to have this output:
ITEMID | STOCK | DOCDATE | QTY
------------------------------
Test1 | 10 |2/28/2017 | 5
Test2 | 15 |2/28/2017 | 8
Test3 | 12 |2/28/2017 | 6

Answer 1

If you only want to consolidate rows coming from the two different tables then you can use this query:
select itemid, MAX(stock), MAX(docdate), MAX(qty)
from (
select itemid,stock,null docdate,0 qty from stk
union
select itemid,0 stock,docdate,qty from qty
) as t
group by itemid
order by 1
Demo here

Answer 2

If your only goal is to have that output, then you can just do this:
SELECT Q.ItemId, (SELECT S.stock FROM stk S ON S.ItemId = Q.ItemId) AS STOCK, Q.DocDate, Q.Qty FROM qty Q
I am using subqueries to connect the two tables on Id, and then I select the stock from Stk where the ItemId is the same as the ItemId from Qty.

Answer 3

Unions are vertical joins are horizontal. Perhaps something like this is required.
MariaDB [sandbox]> create table stk(ITEMID varchar(10), STOCK int);
Query OK, 0 rows affected (0.20 sec)
MariaDB [sandbox]> insert into stk values
-> ('Test1' , 10),
-> ('Test2' , 15),
-> ('Test3' , 12);
Query OK, 3 rows affected (0.02 sec)
Records: 3 Duplicates: 0 Warnings: 0
MariaDB [sandbox]>
MariaDB [sandbox]> drop table if exists qty;
Query OK, 0 rows affected (0.14 sec)
MariaDB [sandbox]> create table qty(ITEMID varchar(10), DOCDATE varchar(10) , QTY int);
Query OK, 0 rows affected (0.20 sec)
MariaDB [sandbox]> insert into qty values
-> ('Test1' , '2/28/2017' , 5),
-> ('Test2' , '2/28/2017' , 8),
-> ('Test3' , '2/28/2017' , 6),
-> ('Test1' , '3/28/2017' , 7),
-> ('Test2' , '3/28/2017' , 7),
-> ('Test3' , '3/28/2017' , 7);
Query OK, 6 rows affected (0.01 sec)
Records: 6 Duplicates: 0 Warnings: 0
MariaDB [sandbox]>
MariaDB [sandbox]> select s.itemid,s.stock,
-> q.docdate,sum(q.qty) qty
-> from stk s
-> join qty q on q.itemid = s.itemid
-> group by s.itemid,s.stock,q.docdate
-> order by q.docdate, s.itemid,s.stock
->
-> ;
+--------+-------+-----------+------+
| itemid | stock | docdate | qty |
+--------+-------+-----------+------+
| Test1 | 10 | 2/28/2017 | 5 |
| Test2 | 15 | 2/28/2017 | 8 |
| Test3 | 12 | 2/28/2017 | 6 |
| Test1 | 10 | 3/28/2017 | 7 |
| Test2 | 15 | 3/28/2017 | 7 |
| Test3 | 12 | 3/28/2017 | 7 |
+--------+-------+-----------+------+
6 rows in set (0.00 sec)

Answer 4

Use JOIN to get result:
SELECT ITEMID ,STOCK,DOCDATE,QTY
FROM table1
JOIN table2 ON table1.ITEMID = table2.ITEMID