Building extended properties from existing table - sql-server

Question

I need to document metadata for the fields of multiple tables. I intend to do this by appending the tables with extended properties. The only way I've done this in the past is using the built-in stored procedure which looks like this:
ALTER procedure [sys].[sp_addextendedproperty]
#name sysname,
#value sql_variant = NULL,
#level0type varchar(128) = NULL,
#level0name sysname = NULL,
#level1type varchar(128) = NULL,
#level1name sysname = NULL,
#level2type varchar(128) = NULL,
#level2name sysname = NULL
as
declare #ret int
if datalength(#value) > 7500
begin
raiserror(15097,-1,-1)
return 1
end
if #name is null
begin
raiserror(15600,-1,-1,'sp_addextendedproperty')
return (1)
end
execute #ret = sys.sp_validname #name
if (#ret <> 0)
begin
raiserror(15600,-1,-1,'sp_addextendedproperty')
return (1)
end
BEGIN TRANSACTION
begin
EXEC %%ExtendedPropertySet().AddValue(Name = #name, Value =
#value, Level0type = #level0type, Level0name = #level0name, Level1type =
#level1type, Level1name = #level1name, Level2type = #level2type,
Level2name = #level2name)
IF ##error <> 0
begin
COMMIT TRANSACTION
return (1)
end
end
COMMIT TRANSACTION
return (0)
The way I've done this in the past is by just passing strings to the variables:
EXEC sys.sp_addextendedproperty
#name=N'desc_en',
#value=N'Sex. 1 denotes male, 2 denotes female.' ,
#level0type=N'SCHEMA',
#level0name=N'dbo',
#level1type=N'TABLE',
#level1name=N'MyTable',
#level2type=N'COLUMN',
#level2name=N'sex'
GO
But now that I have to do this for multiple tables (and ideally write some re-usable procedure), I'm attempting to write a function that takes dynamic (i.e. SELECT) arguments.
For instance, take the following tables. The first has a couple of fields that need extended properties, whereas the other table has two fields containing that information:
dbo.users:
id | name | sex | year
-------------------------
1 | 'john' | 1 | 2019
2 | 'jane' | 2 | 2019
dbo.metadata:
id | property_en | desc_en
-------------------------
1 | sex | Sex. 1 denotes male, 2 denotes female
2 | year | The year of the event record
desired ep:
EXEC sys.sp_addextendedproperty
#name=N'desc_en',
#value=N'Sex. 1 denotes male, 2 denotes female' ,
#level0type=N'SCHEMA',
#level0name=N'dbo',
#level1type=N'TABLE',
#level1name=N'users',
#level2type=N'COLUMN',
#level2name=N'sex'
Any suggestions to derive EPs directly from a source and metadata table like that as an iteration? The first set of arguments have to come from the source table itself (i.e., information about the schema, table name and column name), whereas the second from the metadata table itself (i.e. property_en and desc_en).
Essentially, the first table (t1) provides:
#level0name
#level1name
And t2:
#name
#value
And the join is on #level2name, in other words where t1.clmns.name = t2.property_en

Answer 1

Here we go. Dynamic sql like this is a bit tricky if you haven't done it very much. In essence I am just using the system tables to get the information needed based on the data in your metadata table. Then we just build up a big sql string with a bunch of stored proc calls in it. Finally we just fire it off to run.
Please note, this will find any column that matches in any table based on the name in metadata. If you need to refine that for something like only tables with both columns or something you will need to adjust this a little bit.
create table metadata
(
id int
, property_en sysname
, desc_en varchar(500)
)
insert metadata values
(1, 'sex', 'Sex. 1 denotes male, 2 denotes female')
, (2, 'year', 'The year of the event record')
declare #SQL nvarchar(max) = ''
select #SQL = #SQL + 'EXEC sys.sp_addextendedproperty #name=N''' + d.property_en + ''', #value=N''' + d.desc_en + ''', #level0type=N''SCHEMA'', #level0name=N''' + s.name + ''', #level1type=N''TABLE'', #level1name=N''' + t.name + ''', #level2type=N''COLUMN'', #level2name=N''' + d.property_en + ''';'
from metadata d
join sys.columns c on c.name = d.property_en
join sys.tables t on t.object_id = c.object_id
join sys.schemas s on s.schema_id = t.schema_id
select #SQL
--once you are comfortable that the dynamic sql works uncomment the line below.
--exec sp_executesql #SQL

Answer 2

Here is an 'instead of' trigger based mechanism based on prior work by Cade Roux and "Phil Factor" (credits in the comments)
https://github.com/phrrngtn/rule4/blob/main/sql/extended_properties.sql
This creates some views RULE4.extended_property and RULE4.extended_property_aux that you can treat as normal table and insert/update/delete extended properties. The instead of trigger proxies this onto calls to the corresponding extended properties sproc.