How to retrieve data from multiple daily tables in SQL Server - sql

Question

I am new to Microsoft SQL Server 2008. I need some help in retrieving data from multiple tables which have same columns with same datatype.
Currently I have multiple tables in my database, each table contains data for a single day. For example, I have the following tables:
Table_20160820
Table_20160821
Table_20160822
Table_20160823
Table_20160824
Table_20160825
All the tables have same columns with same datatype say:
column_1
column_2
column_3
column_4
Now, is it possible to retrieve the columns from all the tables using a single query.
What I am exactly looking for is to get the count of column_3 which will be grouped by column_1 for all the tables. I want to avoid writing multiple queries as I need to fetch the data for an entire month i.e. from 30 tables.

Answer 1

As the comments have said, it'd be best to organize this into a single table, and then add a date column so you can see when each row was added. You can even set the default value of the date_created column to be getdate().
To get all of the data together use the UNION Operator in conjunction with an insert statement to your new better table.
CREATE TABLE Better_Table (Column_1 <type>, Column_2 <type>, ..., Date_Created DateTime2(7) Default GETDATE())
GO
INSERT INTO Better_Table (Column_1, Column_2, ...)
SELECT *, '2016/08/22' FROM Table_20160822
UNION
SELECT *, '2016/08/23' FROM Table_20160823
...
Once you've done this your query will be far simpler:
SELECT Column_1, COUNT(Column_3)
FROM Better_Table
WHERE date_created >= '2016/08/01'
AND date_created < '2016/09/01'
GROUP BY Column_1
Although you can still do this in the db current state by using the union operator in the group by query. It would look something like this:
SELECT a.column_1, COUNT(a.column_3)
FROM (
SELECT column_1, Column_3
FROM Table_20160822
UNION ALL
SELECT column_1, Column_3
FROM Table_20160823
) a
GROUP BY a.Column_1
Just expand this to the tables you need to grab from.

Answer 2

You can use CTE to get all dates you need, then create a query and execute it:
DECLARE #dateStart datetime = '2016-08-01',
#dateFinish datetime = '2016-09-01',
#query nvarchar(max)
;WITH cte as (
SELECT #dateStart as d
UNION ALL
SELECT DATEADD(day,1,d)
FROM cte
WHERE DATEADD(day,1,d) < #dateFinish
)
SELECT #query = (
SELECT 'SELECT column_1, COUNT(column_3) FROM ( '
+ STUFF((
SELECT ' UNION ALL SELECT * FROM [Table_'+CONVERT(nvarchar(8),d,112)+'] '
FROM cte
FOR XML PATH('')
),1,10,'') + ') as t GROUP BY column_1 '
FOR XML PATH('')
)
PRINT #query
EXEC sp_executesql #query
Output of #query variable:
SELECT column_1, COUNT(column_3)
FROM (
SELECT * FROM [Table_20160801] UNION ALL
SELECT * FROM [Table_20160802] UNION ALL
SELECT * FROM [Table_20160803] UNION ALL
SELECT * FROM [Table_20160804] UNION ALL
....
SELECT * FROM [Table_20160831]
) as t
GROUP BY column_1

Answer 3

As mentioned above this is a bad design and it would be good if the design is fixed at first place. This is a technical debt which has to be paid back sooner or later. Anyway here's how you can do it:
DECLARE #dtStartDate DATE,
#dtEndDate DATE,
#nvchTablePrefix NVARCHAR(50),
#nvchQuery NVARCHAR(max)
SELECT #dtStartDate = '20160801',
#dtEndDate = '20160831',
#nvchTablePrefix = 'Table_'
CREATE TABLE #temp
(name NVARCHAR(100))
WHILE(#dtStartDate <= #dtEndDate)
BEGIN
INSERT INTO #temp
SELECT ' SELECT column_1, column_3 FROM ' + #nvchTablePrefix + CONVERT(NVARCHAR,#dtStartDate,112)
SELECT #dtStartDate = DATEADD(d,1,#dtStartDate)
IF (#dtStartDate <= #dtEndDate)
INSERT INTO #temp
SELECT ' UNION '
END
SELECT #nvchQuery = 'SELECT column_1, COUNT(column_3) as column3count FROM ( '
SELECT #nvchQuery = #nvchQuery + name FROM #temp
SELECT #nvchQuery = #nvchQuery + ') t GROUP BY column1'
PRINT #nvchQuery
EXEC sp_executesql #nvchQuery
DROP TABLE #temp
OUTPUT of print statement will be like -
SELECT column_1, COUNT(column_3) as column3count
FROM
( SELECT column_1, column_3 FROM Table_20160801
UNION
SELECT column_1, column_3 FROM Table_20160802
UNION
....
UNION
SELECT column_1, column_3 FROM Table_20160831
) t GROUP BY column1