Union Query of Date Fields in MS Access 2010

Question

I have entries in a data table TableofDates comprising three fields; ProjectID (type = dbText), Start (type = dbDate) and Finish (type = dbDate):

TableofDates

ProjectID   Start       Finish  
ABC         01/01/2018  09/09/2018

I wish to present the following dataset with individual DateA records (derived from [Start] in [TableofDates]) and DateB records (derived form [Finish] in [TableofDates])

ToD UNION

ProjectID   DateA       DateB
ABC         01/01/2018  
ABC                     09/09/2018

The following SQL Query produces almost the result required:

SELECT ProjectID,DateA, '' AS DateB
FROM DtA
UNION SELECT ProjectID, '' AS DateA,DateB
FROM DtB;

The problem I have is that the inserted null strings force the field data to type dbText.

If instead I insert Nulls, I get only the first date field reproduced:

SELECT ProjectID, DateA, Null AS DateB 
FROM DtA 
UNION SELECT ProjectID, Null AS DateA, DateB 
FROM DtB;

yields:

ToD UNION
ProjectID   DateA       DateB
ABC         01/01/2018  
ABC     

Both parts of the Union work individually:

SELECT ProjectID,DateA, Null AS DateB
FROM DtA;

yields

ToD UNION
ProjectID   DateA        DateB
ABC         01/01/2018  

While

SELECT ProjectID,Null AS DateA,  DateB
FROM DtB;

Yields

ToD UNION
ProjectID   DateA       DateB
ABC                     09/09/2018

Any ideas how I can create a union of TableofDates with inserted "Null" dates as required, and maintain Data Type of dbDate in the resultant dataset?

Answer 1

You can use a header row (a select statement that returns 0 records, but includes the correct types and labels)

SELECT CLng(1) As ProjectID, #2001-01-01# As DateA, #2001-01-01# As DateB
FROM MSysObjects
WHERE 1=0
UNION ALL
SELECT ProjectID, DateA, Null 
FROM DtA 
UNION ALL
SELECT ProjectID, Null, DateB 
FROM DtB;

Your query doesn't work, because in Access, the first query of a union query determines the type and label of the field. Because the last column of the first query only contains Null , Access can't determine a field type, and chooses the wrong one.

By using a query that doesn't return records, but sets the field type explicitly, you're avoiding that problem.

Note that the change from UNION (which typecasts and checks for duplicates) to UNION ALL (which doesn't do those things) also fixes the problem. So 2 solutions for the price of one.

Answer 2

A Union query will ignore field alias names except for those in the first SELECT statement.

A NULL value is less than any date value, so a descending sort will place the NULLS first.

This SQL produces your example output:

SELECT              ProjectID, Start AS DateA, NULL AS DateB
FROM                TableOfDates

UNION ALL SELECT    ProjectID, NULL, Finish
FROM                TableOfDates
ORDER BY            ProjectID, DateA DESC  

This table:

ProjectID   Start       Finish  
ABC         01/01/2018  09/09/2018  
DEF         01/02/2017  03/05/2018  

Produced this result:

ProjectID   DateA       DateB  
ABC         01/01/2018    
ABC                     09/09/2018  
DEF         01/02/2017  
DEF                     03/05/2018