What is the encode(<columnName>, 'escape') PostgreSQL equivalent in SQL Server? - sql-server

Question

In the same vein as this question, what is the equivalent in SQL Server to the following Postgres statement?
select encode(some_field, 'escape') from only some_table

Answer 1

As you were told already, SQL-Server is not the best with such issues.
The most important advise to avoid such issues is: Use the appropriate data type to store your values. Storing binary data as a HEX-string is running against this best practice. But there are some workarounds:
I use the HEX-string taken from the linked question:
DECLARE #str VARCHAR(100)='0x61736461640061736461736400';
--here I use dynamically created SQL to get the HEX-string as a real binary:
DECLARE #convBin VARBINARY(MAX);
DECLARE #cmd NVARCHAR(MAX)=N'SELECT #bin=' + #str;
EXEC sp_executeSql #cmd
,N'#bin VARBINARY(MAX) OUTPUT'
,#bin=#convBin OUTPUT;
--This real binary can be converted to a VARCHAR(MAX).
--Be aware, that in this case the input contains 00 as this is an array.
--It is possible to split the input at the 00s, but this is going to far...
SELECT #convBin AS HexStringAsRealBinary
,CAST(#convBin AS VARCHAR(MAX)) AS CastedToString; --You will see the first "asda" only
--If your HEX-string is not longer than 10 bytes there is an undocumented function:
--You'll see, that the final AA is cut away, while a shorter string would be filled with zeros.
SELECT sys.fn_cdc_hexstrtobin('0x00112233445566778899AA')
SELECT CAST(sys.fn_cdc_hexstrtobin(#str) AS VARCHAR(100));
UPDATE: An inlinable approach
The following recursive CTE will read the HEX-string character by character.
Furthermore it will group the result and return two rows in this case.
This solution is very specific to the given input.
DECLARE #str VARCHAR(100)='0x61736461640061736461736400';
WITH recCTE AS
(
SELECT 1 AS position
,1 AS GroupingKey
,SUBSTRING(#str,3,2) AS HEXCode
,CHAR(SUBSTRING(sys.fn_cdc_hexstrtobin('0x' + SUBSTRING(#str,3,2)),1,1)) AS TheLetter
UNION ALL
SELECT r.position+1
,r.GroupingKey + CASE WHEN SUBSTRING(#str,2+(r.position)*2+1,2)='00' THEN 1 ELSE 0 END
,SUBSTRING(#str,2+(r.position)*2+1,2)
,CHAR(SUBSTRING(sys.fn_cdc_hexstrtobin('0x' + SUBSTRING(#str,2+(r.position)*2+1,2)),1,1)) AS TheLetter
FROM recCTE r
WHERE position<LEN(#str)/2
)
SELECT r.GroupingKey
,(
SELECT x.TheLetter AS [*]
FROM recCTE x
WHERE x.GroupingKey=r.GroupingKey
AND x.HEXCode<>'00'
AND LEN(x.HEXCode)>0
ORDER BY x.position
FOR XML PATH(''),TYPE
).value('.','varchar(max)')
FROM recCTE r
GROUP BY r.GroupingKey;
The result
1 asdad
2 asdasd
Hint: Starting with SQL Server 2017 there is STRING_AGG(), which would reduce the final SELECT...

Answer 2

If you need this functionality, it's going to be up to you to implement it. Assuming you just need the escape variant, you can try to implement it as a T-SQL UDF. But pulling strings apart, working character by character and building up a new string just isn't a T-SQL strength. You'd be looking at a WHILE loop to count over the length of the input byte length, SUBSTRING to extract the individual bytes, and CHAR to directly convert the bytes that don't need to be octal encoded.1
If you're going to start down this route (and especially if you want to support the other formats), I'd be looking at using the CLR support in SQL Server, to create the function in a .NET language (C# usually preferred) and use the richer string manipulation functionality there.
Both of the above assume that what you're really wanting is to replicate the escape format of encode. If you just want "take this binary data and give me a safe string to represent it", just use CONVERT to get the binary hex encoded.
1Here's my attempt at it. I'd suggest a lot of testing and tweaking before you use it in anger:
create function Postgresql_encode_escape (#input varbinary(max))
returns varchar(max)
as
begin
declare #i int
declare #len int
declare #out varchar(max)
declare #chr int
select #i = 1, #out = '',#len = DATALENGTH(#input)
while #i <= #len
begin
set #chr = SUBSTRING(#input,#i,1)
if #chr > 31 and #chr < 128
begin
set #out = #out + CHAR(#chr)
end
else
begin
set #out = #out + '\' +
RIGHT('000' + CONVERT(varchar(3),
(#chr / 64)*100 +
((#chr / 8)%8)*10 +
(#chr % 8))
,3)
end
set #i = #i + 1
end
return #out
end