建立 MySQL 函數以從 SELECT 結果傳回值

Question

我想在 mySql 8 上建立此函數。它將建立一個類似 00001,00002 的序號

CREATE FUNCTION dbOne.create_sequence_number(lastNumber CHAR(255), numberLength INT, lastValue CHAR(255) ) RETURNS char(255)
BEGIN
    DECLARE select_var CHAR(255);
    SET select_var = (SELECT 
        CASE WHEN lastNumber = lastValue 
        THEN
        LPAD( '1', numberLength, '0' ) 
        ELSE 
        LPAD(CAST(( CAST(COALESCE ( lastNumber, '0' ) AS INT) + 1 ) AS VARCHAR, numberLength, '0' ) INTO select_var);
    RETURN select_var;
END

我不知道這個查詢有什麼問題，但我總是收到這個錯誤。

SQL Error [1064] [42000]: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'INT) + 1 ) AS VARCHAR, numberLength, '0' ) INTO select_var);
    RETURN select_var' at line 9
  You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'INT) + 1 ) AS VARCHAR, numberLength, '0' ) INTO select_var);
    RETURN select_var' at line 9
  You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'INT) + 1 ) AS VARCHAR, numberLength, '0' ) INTO select_var);
    RETURN select_var' at line 9

我也嘗試過這個查詢。

CREATE FUNCTION erhav2_db.create_sequence_number(lastNumber CHAR(255), numberLength INT, lastValue CHAR(255) ) RETURNS char(255)
BEGIN
    DECLARE select_var CHAR(255);
    SELECT 
        (CASE WHEN lastNumber = lastValue 
        THEN
        lpad( '1', numberLength, '0' ) 
        ELSE 
        lpad(CAST(( CAST(COALESCE ( lastNumber, '0' ) AS INT) + 1 ) AS VARCHAR, numberLength, '0' ))) INTO select_var;
    RETURN select_var;
END

但仍然給我同樣的錯誤。我的函數查詢可能會出現什麼問題？

Answer 1

CREATE FUNCTION dbOne.create_sequence_number(
    lastNumber /* CHAR(255) */ UNSIGNED, 
    numberLength INT, 
    lastValue CHAR(255) 
) 
RETURNS CHAR(255)
RETURN LPAD(CASE WHEN lastNumber = lastValue
                 THEN 1
                 ELSE COALESCE(lastNumber, 0) + 1
                 END,
            numberLength, 
            '0');

多次資料類型轉換是多餘的 - MySQL 會根據操作上下文隱式變更資料類型。

所有運算都可以在單一語句中執行，這使得宣告變數和 BEGIN-END（以及分隔符號重新指派）都變得不必要。

程式碼需要 lastNumber 才能轉換成數字資料型別。如果不是，那麼你和我的程式碼在嚴格 SQL 模式下都會失敗。因此，我建議將 lastNumber CHAR(255) 輸入參數資料類型更改為 UNSIGNED / INT - 這將允許在函數呼叫階段檢測值的不正確性，而不是在函數程式碼中。