如何按情况写出正确的顺序

如何解决如何按情况写出正确的顺序

我有这个问题，如果我正常使用 sql 语句，我的意思是这样的：

SELECT p.id_plecari,p.id_aeroport,p.id_clasa,p.id_avion,p.data,p.ora,c.pret,av.nume,a.Nume,a.Locatie
    from plecari p,clasa c,avioane av,aeroport a
where 
   p.id_aeroport = a.id_aeroport
   AND p.id_clasa = c.id_clasa
   AND p.id_avion = av.id_avion
   AND p.id_plecari not in(SELECT r.id_plecare from rezervari r WHERE r.id_client = idClient) 
   ORDER BY p.id_clasa ASC;

一切都如我所愿，完美如this picture
但是在我添加 CASE 语句的那一刻，一切都搞砸了：

BEGIN
SELECT p.id_plecari,aeroport a
    where 
        p.id_aeroport = a.id_aeroport
        AND p.id_clasa = c.id_clasa
        AND p.id_avion = av.id_avion
        AND p.id_plecari not in(SELECT r.id_plecare from rezervari r WHERE r.id_client = idClient) ORDER BY 
        (CASE 
    WHEN (orderby = 1 AND orderas = false) THEN p.id_plecari
    WHEN (orderby = 2 AND orderas = false) THEN av.nume
  END) ASC,(CASE 
    WHEN (orderby = 1 AND orderas = true) THEN p.id_plecari
    WHEN (orderby = 2 AND orderas = true) THEN av.nume
  END) DESC;
END

一切看起来都like this！

解决方法

SELECT
  p.id_plecari,p.id_aeroport,p.id_clasa,p.id_avion,p.data,p.ora,c.pret,av.nume,a.Nume,a.Locatie
FROM
  plecari   p
INNER JOIN
  clasa     c
    ON p.id_clasa = c.id_clasa
INNER JOIN
  avioane   av
    ON p.id_avion = av.id_avion
INNER JOIN
  aeroport  a
    ON p.id_aeroport = a.id_aeroport
WHERE
  p.id_plecari not in (
    SELECT r.id_plecare from rezervari r WHERE r.id_client = idClient
  )
ORDER BY 
  CASE WHEN (orderby = 1 AND orderas = false) THEN p.id_plecari END,CASE WHEN (orderby = 2 AND orderas = false) THEN av.nume      END,CASE WHEN (orderby = 1 AND orderas = true ) THEN p.id_plecari END DESC,CASE WHEN (orderby = 2 AND orderas = true ) THEN av.nume      END DESC;

这显示了如何使用 JOIN，以及如何将您的 ORDER BY 分解成多个部分。

但是请注意，这种“条件排序”样式的性能可能非常差。

这是因为数据库必须生成一个单一的执行计划来满足所有可能性。

更高性能的方法是将主查询作为视图，然后使用类似于...的东西

IF (orderby = 1 AND orderas = false) THEN
  SELECT * FROM yourView ORDER BY p.id_plecari;
ELSEIF (orderby = 2 AND orderas = false) THEN
  SELECT * FROM yourView ORDER BY av.nume;
ELSEIF (orderby = 1 AND orderas = true) THEN
  SELECT * FROM yourView ORDER BY p.id_plecari DESC;
ELSEIF (orderby = 2 AND orderas = true) THEN
  SELECT * FROM yourView ORDER BY av.nume DESC;
ENDIF;

这允许每个排序模式有不同的执行计划。

• This 更详细地解释了