公用表表達式 - 遞歸調用

Question

我在這里找到了這段代碼

WITH ShowMessage(STATEMENT, LENGTH)
AS
(
SELECT STATEMENT = CAST('I Like ' AS VARCHAR(300)), LEN('I Like ')
UNION ALL
SELECT
CAST(STATEMENT + 'CodeProject! ' AS VARCHAR(300))
, LEN(STATEMENT) FROM ShowMessage
WHERE LENGTH < 300
)
SELECT STATEMENT FROM ShowMessage

輸出：

我無法正確理解代碼的含義，尤其是長度的使用。

Answer 1

當您將其更改為包含LENGTH列時，請查看查詢結果。

WITH ShowMessage(STATEMENT, LENGTH)
AS
(
    SELECT  STATEMENT = CAST('I Like ' AS VARCHAR(300)), 
            LEN('I Like ')
    UNION ALL
    SELECT  CAST(STATEMENT + 'CodeProject! ' AS VARCHAR(300)), 
            LEN(STATEMENT) FROM ShowMessage
    WHERE   LENGTH < 300
)
SELECT  STATEMENT,
        LENGTH
FROM    ShowMessage

您會注意到每次遞歸時字符串的長度變長。

一旦leng增長到300以上，遞歸循環將結束