MySQL总结一些CASE WHEN函数

Question

当我期望任何 CASE 都有三个不同的表达式时，是否有可能将三个 CASE 函数最小化为一个？

因此，所有 CASE 调用都依赖于一个值 (con.category_id)。我希望您能帮助我通过保留整个功能来清理此 SQL 语句。

SELECT 
 UNIX_TIMESTAMP(con.content_date_publishing) AS time, 
 CASE 
  when con.category_id IS NOT NULL 
 THEN 
  con.category_id 
 ELSE 
  0 
 END AS category_id, 
 CASE 
  when con.category_id IS NOT NULL 
 THEN 
  cat.category_name_de 
 ELSE 
  NULL 
 END AS category_name 
FROM 
 v5_content AS con, 
 v5_category AS cat 
WHERE content_id = 2 AND 
 CASE 
  when con.category_id IS NOT NULL 
 THEN 
  con.category_id = cat.category_id 
 ELSE 
  1 

END 
LIMIT 
 1

Answer 1

您似乎缺少 LEFT JOIN 运算符。

SELECT
    UNIX_TIMESTAMP(con.content_date_publishing) AS time,
    cat.category_id,
    cat.category_name_de
FROM v5_content as con
LEFT JOIN v5 category as cat ON con.category_id = cat.category_id
WHERE
    content_id = 2;