MySQL selct multiple column from multiple tables with no link - mysql

Question

Table1
id
hour
date
tableValue1
tableValue2
1
3
2020-05-29
123
145
2
2
2020-05-29
1500
3400
Table2:
id
hour
date
tableValue3
tableValue4
1
1
2020-05-29
4545
3697
2
3
2020-05-29
5698
2896
Table3:
id
hour
date
tableValue5
tableValue6
1
2
2020-05-29
7841
5879
2
1
2020-05-29
1485
3987
I want to select multiple columns from different tables with one query.
Expected Output:
hour
tableValue1
tableValue3
tableValue5
1
0
4545
1485
2
1500
0
7841
3
123
5698
0
I've tried this query without success:
SELECT hour , tableValue1 WHERE date = "2020-05-29" AND hour BETWEEN 0 AND 10 FROM table1
UNION ALL
SELECT hour , tableValue3 WHERE date = "2020-05-29" AND hour BETWEEN 0 AND 10 FROM table2
UNION ALL
SELECT hour , tableValue5 WHERE date = "2020-05-29" AND hour BETWEEN 10 AND 10 FROM table3
I'm getting instead the following:
hour
tableValue1
3
123
2
1500
1
4545
3
5698
2
5879
1
3987
The columns tables have in common are hour and date, do I need to redesign database structure to link the tables, so that I can use JOIN command, but how? Or is there a sql command to select multiple column from multiple tables?

Answer 1

There are a couple of issues in your code:
your WHERE clause should be found after the FROM clause in your subqueries
you want different columns, but you associate only one column for each of your table: if you want three columns, each of your subqueries should return three columns
your rows are not ordered because you're missing an ORDER BY clause at the end of your code.
your rows are not aggregated to remove the zeroes in excess: in that case it is sufficient to apply a MAX aggregation function for each relevant field, partitioning on the "hour" field
WITH cte AS (
SELECT hour,
tableValue1,
0 AS tableValue3,
0 AS tableValue5
FROM table1
WHERE date = "2020-05-29" AND hour BETWEEN 0 AND 10
UNION ALL
SELECT hour,
0 AS tableValue1,
tableValue3,
0 AS tableValue5
FROM table2
WHERE date = "2020-05-29" AND hour BETWEEN 0 AND 10
UNION ALL
SELECT hour,
0 AS tableValue1,
0 AS tableValue3,
tableValue5
FROM table3
WHERE date = "2020-05-29" AND hour BETWEEN 0 AND 10
ORDER BY hour
)
SELECT hour,
MAX(tableValue1) AS tableValue1,
MAX(tableValue3) AS tableValue3,
MAX(tableValue5) AS tableValue5
FROM cte
GROUP BY hour
Check the demo here.

Answer 2

You must introduce empty columns in first query
SELECT hour , tableValue1, 0 tableValue3, 0 tableValue5 FROM table1 WHERE date = "2020-05-29" AND hour BETWEEN 0 AND 10
UNION ALL
SELECT hour , 0, tableValue3, 0 FROM table2 WHERE date = "2020-05-29" AND hour BETWEEN 0 AND 10
UNION ALL
SELECT hour , 0,0 tableValue5 FROM table3 WHERE date = "2020-05-29" AND hour BETWEEN 10 AND 10