MySQL：选择字符串的连接和结果连接的长度

Question

In my CREATE VIEW I would like to:

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

but this ill produce error:

Unknown column 'Title' in 'field list'.

What is the correct way of doing this without having to Concat same strings twice?

Answer 1

You cannot refer the alias you created in SELECT , use expression instead:

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

You can use subquery if you need:

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.

and:

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.