PL/SQL塊的結構主要包括四個部分：DECLARE、BEGIN、EXCEPTION和END。 1. DECLARE是可選部分，用於聲明變量、常量、游標和用戶定義類型，所有聲明必須位於BEGIN關鍵字之前；2. BEGIN是必填部分，包含執(zhí)行邏輯，如SQL語句、控制結構和函數(shù)調用，是實際工作完成的地方；3. EXCEPTION是可選的錯誤處理部分，用於處理運行時錯誤，支持特定異常（如NO_DATA_FOUND）和通用異常（WHEN OTHERS THEN）；4. END標記塊的結束，每個PL/SQL塊都必須以END關鍵字及分號結尾，必要時可為塊添加標籤以增強可讀性。

Sure! A PL/SQL block is the basic unit of code in Oracle's PL/SQL language. It allows you to group logically related statements together and execute them as a single unit. The structure of a PL/SQL block typically includes four sections: DECLARE, BEGIN, EXCEPTION, and END.

1. DECLARE (Optional) – For Variable and Cursor Declarations

This section is where you declare variables, constants, cursors, and user-defined types that will be used in the executable part of the block.

• You don't have to include this section if your block doesn't need any variables.
• All declarations must come before the BEGIN keyword.

Example:

 DECLARE
   v_salary NUMBER(10,2);
   c_tax_rate CONSTANT NUMBER := 0.15;

These variables can then be used inside the BEGIN section.

2. BEGIN – The Executable Section (Required)

This is the only required part of a PL/SQL block. It contains the logic — SQL statements, control structures (like IF-THEN-ELSE or loops), function calls, etc.

• This is where the actual work gets done.
• Statements are executed sequentially unless controlled by logic.

Example:

 BEGIN
   SELECT salary INTO v_salary FROM employees WHERE employee_id = 100;
   v_salary := v_salary * (1 c_raise_percent);
   DBMS_OUTPUT.PUT_LINE('New salary: ' || v_salary);

Even if you don't need to declare anything, you still need the BEGIN section for execution.

3. EXCEPTION (Optional) – Error Handling

This section handles runtime errors (exceptions) that occur during execution.

• If an error occurs and isn't handled, the program stops and returns the error to the calling environment.
• You can define handlers for specific exceptions like NO_DATA_FOUND , TOO_MANY_ROWS , or use WHEN OTHERS THEN as a catch-all.

Common exceptions to handle:

• NO_DATA_FOUND : When a SELECT INTO statement returns no rows.
• TOO_MANY_ROWS : When a SELECT INTO returns more than one row.
• Custom exceptions can also be raised and handled.

Example:

 EXCEPTION
   WHEN NO_DATA_FOUND THEN
      DBMS_OUTPUT.PUT_LINE('No employee found with that ID.');
   WHEN OTHERS THEN
      DBMS_OUTPUT.PUT_LINE('An unexpected error occurred.');

Including proper exception handling makes your code more robust and easier to debug.

4. END – Marks the End of the Block

Every PL/SQL block must end with the END keyword followed by a semicolon ( ; ).

• Think of it like closing a chapter — it tells the compiler that the block is complete.
• Named blocks can include a label after END for clarity in nested or complex code:

 END salary_calculation;

So, putting it all together:

 DECLARE
   v_salary NUMBER(10,2);
   c_raise_percent NUMBER := 0.10;
BEGIN
   SELECT salary INTO v_salary FROM employees WHERE employee_id = 100;
   v_salary := v_salary * (1 c_raise_percent);
   DBMS_OUTPUT.PUT_LINE('New salary: ' || v_salary);
EXCEPTION
   WHEN NO_DATA_FOUND THEN
      DBMS_OUTPUT.PUT_LINE('Employee not found.');
   WHEN OTHERS THEN
      DBMS_OUTPUT.PUT_LINE('Error occurred.');
END;

That's basically how a PL/SQL block is structured. Not too bad once you get used to it.

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