How to get text for default language

Question

I have so tables:

and so data at Language table:

and so data at Text table:

I have to return text for requested language if it exists and text for default language if it does not exist. Is it possible to do that in one query (no while, please)?

Code:

DECLARE @CommentId  bigint = 1
--DECLARE @LanguageCode  nvarchar(2) = 'en' -- "english text" returns
DECLARE @LanguageCode  nvarchar(2) = 'ua'   -- nothing at this moment

SELECT
     t.CommentId
    ,t.TextId
    ,t.[Text]
    ,t.LanguageId
    ,RequestedLanguageId = @LanguageCode
FROM dbo.common_Text t 
    INNER JOIN dbo.common_LanguageType l 
        ON t.LanguageId = l.LanguageId
WHERE l.Code = @LanguageCode 
    AND t.CommentId = @CommentId

Thank you.

ADDED:

If code requests a text in 'ua' (ukrainian) and this is no any text for this language, so it is going to search text for russian. if found - ok, if don't it will look for a text for english. List of languages can vary.

Answer 1

Using a recursive CTE, this query builds a table of all languages that point somewhere in their default chain to a language that has the required comment. It shows each language next to its first default that has the required comment. It then filters this table to give you the first available text for the language you selected.

DECLARE @CommentId BIGINT = 1;
DECLARE @LanguageCode NVARCHAR(2) = 'en';

WITH languages AS (
    -- base case: language has required comment
    SELECT 
          lt.LanguageId AS RootLanguageId
        , lt.LanguageId
        , lt.Code
        , lt.DefaultId
        , 0 AS Level
    FROM 
        dbo.common_LanguageType lt
    --WHERE
    --  lt.LanguageId = lt.DefaultId
    WHERE
        EXISTS (
            SELECT *
            FROM dbo.common_Text    t
            WHERE 
                    t.CommentId = @CommentId
                AND t.LanguageId = lt.LanguageId
        )

    UNION ALL

    -- recursive case: language is not its own default and
    --                 does not have the required comment
    SELECT
          l_default.RootLanguageId
        , l.LanguageId
        , l.Code
        , l.DefaultId
        , l_default.Level + 1 AS Level
    FROM
                    dbo.common_LanguageType l
        INNER JOIN  languages               l_default
            ON l.DefaultId = l_default.LanguageId
    WHERE
            l.LanguageId <> l.DefaultId
        AND NOT EXISTS (
            SELECT *
            FROM dbo.common_Text    t
            WHERE 
                    t.CommentId = @CommentId
                AND t.LanguageId = l.LanguageId
        )
)
SELECT t.Text
FROM 
                languages       l
    INNER JOIN  dbo.common_Text t
        ON  l.RootLanguageId = t.LanguageId
WHERE
    l.Code = @LanguageCode
;

Try just running

SELECT *
FROM languages;

if you want to get a feel for what the recursive CTE is doing. I've uploaded a script to create the tables, insert some sample data, and run the above code to gist.

Pretty sexy, no?