MySQL:选择字符串的 Concat 和结果 Concat 的长度

Question

在我的 CREATE VIEW 中，我想:

SELECT CONCAT( t.str1, t.str2 ) AS Title, CHAR_LENGTH( Title ) AS Length

但这会产生错误:

Unknown column 'Title' in 'field list'.

无需连续连接相同字符串两次的正确方法是什么？

Answer 1

您不能引用您在 SELECT 中创建的别名，而是使用表达式:

SELECT CONCAT( t.str1, t.str2 ) AS Title,
       CHAR_LENGTH(CONCAT( t.str1, t.str2 )  ) AS Length
FROM table_name t

如果需要可以使用子查询:

SELECT sub.Title, CHAR_LENGTH( sub.Title ) AS Length
FROM (
   SELECT CONCAT( t.str1, t.str2 ) AS Title
   FROM table_name t
) AS sub;

All-at-once operation :

"All-at-Once Operations" means that all expressions in the same logical query process phase are evaluated logically at the same time.

和:

We cannot use an alias in next column expression in the SELECT clause. In the query we create a corrected first name and we want to use it in next column to produce the full name, but the All-at-Once operations concept tells us you cannot do this because all expressions in the same logical query process phase (here is SELECT) are evaluated logically at the same time.