Need to change GUID to varchar or char - sql-server

Question

I am trying to use a reference table to update a code, but I am getting the error:
Conversion failed when converting from a character string to
uniqueidentifier
The ID is a guid: 086B9FE7-3980-47D7-BB05-003708F1D564 and the reference code I want to use is 4 characters, like H100.
I received the initial error when I tried to alter the datatype in the table and then update the file with the values from the reference table. I have tried converting and casting based on other articles, but even if I cast or convert successfully, I still get the same message.
RefTable:
Id | ReportCode
6340FCEA-161C-42F4-8D7F-46B4C2E6C4E2 | H100
DataTable:
CauseId
6340FCEA-161C-42F4-8D7F-46B4C2E6C4E2
Code I am using to try and update. The first works, and the second bring the error msg:
select cast(nvarchar(36), ID) as ID
from [dbo].[reftable]
UPDATE dbo.datatable
SET causeid = L.reportcode
FROM dbo.datatable S
join dbo.reftable L on S.causeid = L.id

Answer 1

Uniqueidentifier is a particular column type that holds 36 bytes, with some hyphen characters in the middle. Asigning a string value that doesn't match it's format will always fail when trying to convert it.
-- Conversion failed when converting from a character string to uniqueidentifier.
SELECT CONVERT(UNIQUEIDENTIFIER, 'H100')
-- OK
SELECT CONVERT(UNIQUEIDENTIFIER, 'c029f8be-29dc-41c1-8b38-737b4cc5a4df')
If you want to use a common VARCHAR as your new causeid value, you need to first convert the column type to VARCHAR (or NVARCHAR).
ALTER TABLE dbo.datatable ALTER COLUMN causeid VARCHAR(200) -- NOT NULL if you need
If you can't alter the data type it's probably because there is an INDEX or a CONSTRAINT linked to it. When you try to alter, the SQL engine will tell you which object is linked to it. You will have to drop them, alter the column type and then create them again.
I use these queries to check indexes and constraints. I updated the search values for your table.
DECLARE #table_name VARCHAR(200) = 'datatable'
DECLARE #column_name VARCHAR(200) = 'causeid'
-- Indexes
SELECT
SchemaName = SCHEMA_NAME(t.schema_id),
TableName = t.name,
IndexName = ind.name,
IndexType = CASE ind.index_id WHEN 0 THEN 'Heap' WHEN 1 THEN 'Clustered' ELSE 'Nonclustered' END,
Disabled = ind.is_disabled,
ColumnOrder = ic.index_column_id,
ColumnName = col.name,
ColumnType = y.name,
ColumnLength = y.max_length,
ColumnIncluded = ic.is_included_column
FROM
sys.indexes ind
INNER JOIN sys.index_columns ic ON ind.object_id = ic.object_id and ind.index_id = ic.index_id
INNER JOIN sys.columns col ON ic.object_id = col.object_id and ic.column_id = col.column_id
INNER JOIN sys.tables t ON ind.object_id = t.object_id
INNER JOIN sys.types y ON y.user_type_id = col.user_type_id
WHERE
t.is_ms_shipped = 0 AND
t.name = #table_name AND
col.name = #column_name
ORDER BY
SchemaName,
t.name,
ind.name,
ic.index_column_id
-- Constraints
SELECT
TableName = t.Name,
ColumnName = c.Name,
dc.Name,
dc.definition
FROM
sys.tables t
INNER JOIN sys.default_constraints dc ON t.object_id = dc.parent_object_id
INNER JOIN sys.columns c ON dc.parent_object_id = c.object_id AND c.column_id = dc.parent_column_id
WHERE
t.name = #table_name AND
c.name = #column_name
ORDER BY
t.Name