Question

I have two tables and I would like to insert from one into the other. In my staging (source) table every column is defined as nvarchar(300) and this restriction cannot change.

In my destination table, the columns are of all different types. If I want to, for example, select from the source table (data type nvarchar(300)) and insert that column into a data type of decimal(28, 16).

When this happens I get the following error:

Error converting data type nvarchar to numeric.

Even when I use a cast I get the error.

INSERT INTO Destination (
    Weighting
)
VALUES (
    CAST(src.Weighting AS decimal(28, 16))
)

Could null values be affecting this at all? Is there anything else to consider?

Answer 1

If all data in your staging table column can be implicitly converted to the target data type then you do not have to set up an explicit cast.

But if any one value cannot be converted implicitly (i.e. one cell contains a non-numeric or ill-formatted string value that is supposed to end up in a decimal type column) then the entire transaction will fail.

You can migrate the risk of a failing transaction by setting up the insert like this:

INSERT
  LiveTable (
    VarcharCol,
    DecimalCol,
    NonNullableCol
  )
SELECT
  NvarcharCol1,
  CASE ISNUMERIC(nvarcharCol2) = 0 THEN NvarcharColl2 END,
  ISNULL(NvarcharCol3, '')
FROM
  StagingTable

But clearly that means the risk of losing potentially relevant data or numeric precision.

You can read which data types are implicitly convertible between each other on the MSDN (scroll down to the matrix). For all other conversions you’ll have to use CAST or CONVERT.