MySQL的：選擇造成的毗連

Question

在我的CREATE VIEW的字符串和長度的毗連我想：

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

但本病產生錯誤：

Unknown column 'Title' in 'field list'.

是什麼做的正確方法這不必Concat相同的字符串兩次？

Answer 1

您不能引用您在SELECT創建的別名，使用表達式來代替：

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.

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，您可以使用子查詢

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.