Querying Sys.Columns & Sys.Types

If you want to query the structure of a table that includes column names and data types, you have to perform a join between catalog views Sys.Columns and Sys.Types. There are some caveats to this. If any of your columns are defined as nvarchar or user-defined data types, you must qualify the data coming from Sys.Types. When you add a user-defined data type, it is entered into Sys.Types with a reference to it's native data type. In the example of the AdventureWorks database, the data type "AccountNumber" is defined as nvarchar, and shows up in Sys.Types with system_type_id = 231 (which points to "nvarchar", where system_type_id = 231 and user_type_id = 231).


Running the query:


SELECT
Tb.Name,
C.Name,
Tp.Name
FROM Sys.Tables Tb
JOIN Sys.Schemas Sch
ON Sch.Schema_Id = Tb.Schema_Id
JOIN Sys.Columns C
ON C.Object_Id = Tb.Object_Id
JOIN Sys.Types Tp
ON Tp.System_Type_Id = C.System_Type_Id
WHERE Tb.Name = 'Address'
ORDER BY Tb.Name, C.Name, Tp.Name


produces these results:



Weird, huh? Why did 'AddressLine1' show up six times with six different data types? The reason is two-fold. First, 'AddressLine1' is defined as nvarchar(60), which means that it will also show up as "sysname" datatype (think of "sysname" as MicroSoft's built-in user-defined data type).


Take a look at the results of the query below. It shows that, including itself, six different data types are based on nvarchar! That's why 'AddressLine1' showed up six times in the query above.

SELECT Name FROM Sys.Types Tp
WHERE System_Type_Id = 231

Name
-------------------
nvarchar
sysname
AccountNumber
Name
OrderNumber
Phone
(6 row(s) affected)

So let's change our query to use this 'User_Type_Id' column instead:

SELECT
Tb.Name,
C.Name,
Tp.Name
FROM Sys.Tables Tb
JOIN Sys.Schemas Sch
ON Sch.Schema_Id = Tb.Schema_Id
JOIN Sys.Columns C
ON C.Object_Id = Tb.Object_Id
JOIN Sys.Types Tp
ON Tp.User_Type_Id = C.System_Type_Id
WHERE Tb.Name = 'Address'
ORDER BY Tb.Name, C.Name, Tp.Name

This produces the results we want: