mysql - 如果 MySQL CONCAT_WS 中的所有值都为 NULL，如何删除分隔符？

Question

我正在尝试使用 CONCAT_WS 生成主题行。但是，如果值为空，我想忽略分隔符。

请看这个查询

SELECT
CONCAT_WS(" ",
CASE WHEN total_attempts > 0 THEN
CONCAT( call_code_title , " - ", total_attempts, "<sup>",
    CASE WHEN total_attempts = 2 THEN "nd"
    WHEN total_attempts = 3 THEN "rd"
    ELSE "th" END
, "</sup> attempt") ELSE call_code_title END
, "-", program_name) AS callSubject
FROM table

问题是当“program_name”为 NULL 时，字符串末尾总是会出现“-”。我不想连接“-” id program_name IS NULL

我怎么做？

谢谢

Answer 1

SELECT
CONCAT_WS(" ",
CASE WHEN total_attempts > 0 THEN
CONCAT( call_code_title , " - ", total_attempts, "<sup>",
    CASE WHEN total_attempts = 2 THEN "nd"
    WHEN total_attempts = 3 THEN "rd"
    ELSE "th" END
, "</sup> attempt") ELSE call_code_title END
, CONCAT("-", program_name) ) AS callSubject
FROM table

如果 program_name 为 NULL，则 CONCAT("-", program_name) 也将返回 NULL，CONCAT_WS 将通过忽略它所看到的单个 null 参数来正确处理它。

CONCAT()如果任何参数为 NULL，则返回 NULL。

^{— http://dev.mysql.com/doc/refman/5.6/en/string-functions.html#function_concat}

Answer 2

IFNULL(program_name, concat("-", program_name))

SELECT
CONCAT_WS(" ",
CASE WHEN total_attempts > 0 THEN
CONCAT( call_code_title , " - ", total_attempts, "<sup>",
    CASE WHEN total_attempts = 2 THEN "nd"
    WHEN total_attempts = 3 THEN "rd"
    ELSE "th" END
, "</sup> attempt") ELSE call_code_title END
, IFNULL(program_name, concat("-", program_name)) , " " ) AS callSubject
FROM table