Question

就像标题所说的那样。我在if语句中有一个SQL语句（Oracle 9），其值为true。从我记得这样的声明将做SQL Server的预期。在这种情况下，我做错了什么？

IF (SELECT COUNT(*) FROM CUSTOMER_LIST WHERE CUSTOMER='Target')=0 THEN  
INSERT INTO CUSTOMER_LIST (CUSTOMER_ID, CUSTOMER) VALUES
(CUSTOMER_ID_SEQ.nextval, 'Target')

或者更好的是，确保我不插入已存在的数据的适当策略是什么。写入的代码总是插入。这显然不是我想要的，因为这一行已经存在。

当此代码运行时，我收到以下控制台消息。

Error starting at line 1 in command:
IF (SELECT COUNT(*) FROM CUSTOMER_LIST WHERE CUSTOMER='Target')=0 THEN
Error report:
Unknown Command
1 rows inserted.

Answer 1

问题似乎是你正在给PL / SQL预期直接SQL。要么直截了当地说SQL：

INSERT INTO CUSTOMER_LIST (CUSTOMER_ID, CUSTOMER)
SELECT CUSTOMER_ID_SEQ.nextval, 'Target' FROM DUAL
WHERE NOT EXISTS (SELECT 1 FROM CUSTOMER_LIST WHERE CUSTOMER = 'Target')

或者更改周围的上下文，以便Oracle期望PL / SQL。这取决于你如何传递这些代码;但一种可能的方法是将其包裹在BEGIN ... END块中。

此外，您不能将查询用作PL / SQL中的表达式;如果你去PL / SQL路线，你需要将结果存储在某个地方，例如变量。总而言之，你可能有这个：

DECLARE
  v_customer_exists INTEGER;
BEGIN
  SELECT COUNT(*) INTO v_customer_exists FROM CUSTOMER_LIST WHERE CUSTOMER='Target';
  IF v_customer_exists = 0 THEN
    INSERT INTO CUSTOMER_LIST (CUSTOMER_ID, CUSTOMER) VALUES
    (CUSTOMER_ID_SEQ.nextval, 'Target');
  END IF;
END;
/

Answer 2

    DECLARE
       v_count number;

    BEGIN
    SELECT COUNT(*) INTO v_count FROM CUSTOMER_LIST WHERE CUSTOMER='Target';

    IF v_count = 0 THEN
      INSERT INTO CUSTOMER_LIST (CUSTOMER_ID, CUSTOMER) VALUES
(CUSTOMER_ID_SEQ.nextval, 'Target');
    END IF;
    END;

Answer 3

尝试这样的方法来检查是否从选择中返回了任何行。当没有从select：

返回元组时，NOT EXISTS返回true

IF  NOT EXISTS (SELECT * FROM CUSTOMER_LIST WHERE CUSTOMER='Target') THEN  
INSERT INTO CUSTOMER_LIST (CUSTOMER_ID, CUSTOMER) VALUES
(CUSTOMER_ID_SEQ.nextval, 'Target')

COUNT（*）= 0的计算结果为真？

3 个答案: