sql - 每次写一个额外的列来查询不同值的结果

Question

我已经搜索了很长一段时间，但我一直无法找到我所寻找的答案。我有以下查询:

SELECT DISTINCT o.titulo, o.fecha_estreno
FROM Obra o 
WHERE (o.titulo LIKE '%Barcelona%' AND EXISTS(SELECT p.id_obra FROM Pelicula p WHERE p.id_obra = o.id_obra)) OR EXISTS(SELECT DISTINCT pa.id_obra 
FROM Participa pa
WHERE pa.id_obra = o.id_obra AND EXISTS(SELECT DISTINCT l.nombre FROM Lugar l
    WHERE l.nombre LIKE '%Barcelona%' AND EXISTS(SELECT DISTINCT tl.id_lugar FROM TieneLugar tl
        WHERE tl.id_lugar = l.id_lugar AND tl.id_profesional = pa.id_profesional))) OR EXISTS(SELECT DISTINCT er.id_obra 
        FROM EstaRelacionado er 
            WHERE er.id_obra = o.id_obra AND EXISTS(SELECT DISTINCT k.keyword 
            FROM Keywords k
                WHERE k.id_keyword = er.id_keyword AND k.keyword LIKE '%Barcelona%')); 

它基本上做的是在我的数据库中搜索每部电影，这些电影在某种程度上与它所获得的城市相关。我想为每个结果显示第三列，并说明该行作为结果显示的原因(例如:TITLE CONTAINS IT，或 ACTOR FROM THE MOVIE BORN THERE，等等)

感谢您的耐心等待和帮助!

编辑:按照建议，这里有一些输出示例。该列应仅显示与电影相关的第一个原因:

TITULO            FECHA_ESTRENO        CAUSE
----------       ----------------    ----------
Barcelona mia         1967             TITLE

Answer 1

https://www.postgresql.org/docs/7.4/static/functions-conditional.html

The SQL CASE expression is a generic conditional expression, similar to if/else statements in other languages:

CASE WHEN condition THEN result
     [WHEN ...]
     [ELSE result] 
END 

CASE clauses can be used wherever an expression is valid. condition is an expression that returns a boolean result. If the result is true then the value of the CASE expression is the result that follows the condition. If the result is false any subsequent WHEN clauses are searched in the same manner. If no WHEN condition is true then the value of the case expression is the result in the ELSE clause. If the ELSE clause is omitted and no condition matches, the result is null.

您的案例示例:

SELECT (CASE WHEN EXISTS(... l.nombre LIKE '%Barcelona%') THEN 'TITLE CONTAINS IT' WHEN <conditon for actor> THEN 'ACTOR WA BORN THERE' WHEN ... END) as reason