sql - 如果为真，则强制查询结果

Question

我有一个如下所示的查询:

SELECT cust.adm, COUNT(visits.visit_date) as visit_count FROM cust
LEFT JOIN visits ON cust.cust_id = visits.cust_id
WHERE cust.cust_id = 1
GROUP BY cust.adm

结果是:

   adm   |   visit_count
--------------------------
  TRUE   |        0

如果 adm 为真，是否有办法强制“visit_count”的结果为“3”？

Answer 1

您可以使用 case 表达式:

SELECT    cust.adm,
          CASE cust.adm WHEN TRUE THEN 3 ELSE COUNT(visits.visit_date) END AS visit_count 
FROM      cust
LEFT JOIN visits ON cust.cust_id = visits.cust_id
WHERE     cust.cust_id = 1
GROUP BY  cust.adm