CONVERT 與 CAST 都可以 用來做 字符 與 日期 類型之間的轉換。
但是一般來說， 用 CONVERT 比較好， 因為有一個 用于格式化的參數。

你執行一下下面這個 SQL， 就能看出區別出來。

SELECT 0 AS ID , Convert(VARCHAR(30), GETDATE(), 0 ) AS result UNION ALL
SELECT 100 AS ID , Convert(VARCHAR(30), GETDATE(), 100) AS result UNION ALL
SELECT 1 AS ID , Convert(VARCHAR(30), GETDATE(), 1 ) AS result UNION ALL
SELECT 101 AS ID , Convert(VARCHAR(30), GETDATE(), 101) AS result UNION ALL
SELECT 2 AS ID , Convert(VARCHAR(30), GETDATE(), 2 ) AS result UNION ALL
SELECT 102 AS ID , Convert(VARCHAR(30), GETDATE(), 102) AS result UNION ALL
SELECT 2 AS ID , Convert(VARCHAR(30), GETDATE(), 2 ) AS result UNION ALL
SELECT 102 AS ID , Convert(VARCHAR(30), GETDATE(), 102) AS result UNION ALL
SELECT 3 AS ID , Convert(VARCHAR(30), GETDATE(), 3 ) AS result UNION ALL
SELECT 103 AS ID , Convert(VARCHAR(30), GETDATE(), 103) AS result UNION ALL
SELECT 4 AS ID , Convert(VARCHAR(30), GETDATE(), 4 ) AS result UNION ALL
SELECT 104 AS ID , Convert(VARCHAR(30), GETDATE(), 104) AS result UNION ALL
SELECT 5 AS ID , Convert(VARCHAR(30), GETDATE(), 5 ) AS result UNION ALL
SELECT 105 AS ID , Convert(VARCHAR(30), GETDATE(), 105) AS result UNION ALL
SELECT 6 AS ID , Convert(VARCHAR(30), GETDATE(), 6 ) AS result UNION ALL
SELECT 106 AS ID , Convert(VARCHAR(30), GETDATE(), 106) AS result UNION ALL
SELECT 7 AS ID , Convert(VARCHAR(30), GETDATE(), 7 ) AS result UNION ALL
SELECT 107 AS ID , Convert(VARCHAR(30), GETDATE(), 107) AS result UNION ALL
SELECT 8 AS ID , Convert(VARCHAR(30), GETDATE(), 8 ) AS result UNION ALL
SELECT 108 AS ID , Convert(VARCHAR(30), GETDATE(), 108) AS result UNION ALL
SELECT 9 AS ID , Convert(VARCHAR(30), GETDATE(), 9 ) AS result UNION ALL
SELECT 109 AS ID , Convert(VARCHAR(30), GETDATE(), 109) AS result UNION ALL
SELECT 10 AS ID , Convert(VARCHAR(30), GETDATE(), 10 ) AS result UNION ALL
SELECT 110 AS ID , Convert(VARCHAR(30), GETDATE(), 110) AS result UNION ALL
SELECT 11 AS ID , Convert(VARCHAR(30), GETDATE(), 11 ) AS result UNION ALL
SELECT 111 AS ID , Convert(VARCHAR(30), GETDATE(), 111) AS result UNION ALL
SELECT 12 AS ID , Convert(VARCHAR(30), GETDATE(), 12 ) AS result UNION ALL
SELECT 112 AS ID , Convert(VARCHAR(30), GETDATE(), 112) AS result UNION ALL
SELECT 13 AS ID , Convert(VARCHAR(30), GETDATE(), 13 ) AS result UNION ALL
SELECT 113 AS ID , Convert(VARCHAR(30), GETDATE(), 113) AS result UNION ALL
SELECT 14 AS ID , Convert(VARCHAR(30), GETDATE(), 14 ) AS result UNION ALL
SELECT 114 AS ID , Convert(VARCHAR(30), GETDATE(), 114) AS result UNION ALL
SELECT 20 AS ID , Convert(VARCHAR(30), GETDATE(), 20 ) AS result UNION ALL
SELECT 120 AS ID , Convert(VARCHAR(30), GETDATE(), 120) AS result UNION ALL
SELECT 21 AS ID , Convert(VARCHAR(30), GETDATE(), 21 ) AS result UNION ALL
SELECT 121 AS ID , Convert(VARCHAR(30), GETDATE(), 121) AS result