如果变量

Question

假设我有这个问题：

SELECT a.id, 
     c.c_date 
FROM table_a a 
    JOIN table_c c ON a.id = c.id  
WHERE c_date is null;

我还有这个变量： p_filter

当此变量等于'yes'时，我希望先前显示的查询如图所示执行，以便表a和表c连接，并且搜索c_date的空值。如果变量等于'no'或任何其他值，我不想使用table_c而不加入，这样查询就会执行如下：

    SELECT a.id 
    --omitted c.c_date 
FROM table_a a ;
    --omitted JOIN table_c c ON a.id = c.id  
   --omitted  WHERE c_date is null;

Answer 1

也许你可以试试这个;）

SELECT DISTINCT a.id, 
     CASE WHEN $p_filter = 'yes' THEN c.c_date ELSE NULL END AS c_date
FROM table_a a 
JOIN table_c c ON $p_filter = 'no' OR (a.id = c.id AND c.c_date IS NULL);

已编辑：

SELECT DISTINCT a.id, 
     c.c_date
FROM table_a a 
LEFT JOIN table_c c ON $p_filter = 'yes' AND a.id = c.id AND c.c_date IS NULL;

Answer 2

你可以使用union

SELECT a.id, c.c_date
  FROM table_a a JOIN table_c c ON a.id = c.id
 WHERE :p_filter = 'yes' AND c_date IS NULL

UNION ALL

SELECT a.id, NULL c_date
  FROM table_a a
 WHERE NVL (:p_filter, 'no') != 'yes'