Combine values from n-fields with precedence

问题

I have an example table below:

+----+----+------+
|var1|var2|merged|
+----+----+------+
|a   |b   |a     |
+----+----+------+
|    |c   |c     |
+----+----+------+
|    |x   |x     |
+----+----+------+
|c   |d   |c     |
+----+----+------+

I want to merge var1 and var2's values with the rule that whenever var1 has a value, it should be picked over var2.

My code below does not achieve it and I am at a loss on how to implement this.

SELECT id, var1 & var2 AS merged
FROM myTable

All this code does is concatenated var1 and var2's data which is not I want. The one I asked before is not working as desired anymore.

+----+----+------+
|var1|var2|merged|
+----+----+------+
|a   |b   |ab    |
+----+----+------+
|    |c   |c     |
+----+----+------+
|    |x   |x     |
+----+----+------+
|c   |d   |cd    |
+----+----+------+

The one below from this question does not take into account lone values :(


SELECT ID, var1, var2, 
  SWITCH (        
    var1 IS NOT NULL, var1,     
    var2 IS NOT NULL, var2,   
  ) AS merged
FROM myTable;

+----+----+------+
|var1|var2|merged|
+----+----+------+
|a   |b   |a     |
+----+----+------+
|    |c   |      |
+----+----+------+
|    |x   |      |
+----+----+------+
|c   |d   |c     |
+----+----+------+

UPDATE

After using the methods provided in the answers, I have seen some of my records achieve what I presented in the first table above. But after looking through the other rows I managed to find ones that were skipped, as presented in the third table.

Below is the code I use and screenshots of the results.

SELECT ID, var1, var2, 

      # method 1
      SWITCH (        
        var1 IS NOT NULL, var1,     
        var2 IS NOT NULL, var2
      ) AS switchResult,

      # method 2
      Nz(var1, var2) AS NzResult,

      # method 3
      Iif(var1 IS NULL, var2, var1) AS IifResult

FROM myTable;

The results here are what I wanted:

Note: ID, var1, var2, switchResult, NzResult, IifResult are the column names for guide

But as I search for (Blanks) there appears to be failed rows:

It seems that var1's values if var2's missing return correct queries but when var2 does have a value and var1 does not, some rows do not return correct queries.

Does anyone know why this happens? I already checked if var1 and var2 contains whitespace and they do not.

回答1:

You want var1 when it is not Null, and otherwise you want var2. You can use Nz to get that.

SELECT var1, var2, Nz(var1, var2) AS  merged
FROM myTable;

If you will be running the query from outside an Access session, the Nz function will not be available. In that case, you can use an IIf expression.

SELECT var1, var2, IIf(var1 Is Null, var2, var1) AS  merged
FROM myTable;

If var1 may contain zero-length strings (""), and you want those treated the same as Null --- meaning return var2 in that situation --- use this query ...

SELECT var1, var2, IIf(Len(var1 & "") = 0, var2, var1) AS  merged
FROM myTable;

回答2:

try this:

SELECT var1, var2, COALESCE(var1, var2) AS merged
  FROM myTable;

来源：https://stackoverflow.com/questions/21127024/combine-values-from-n-fields-with-precedence

标签

sql

ms-access

ms-access-2007