COUNT(*)= 0的计算结果为真？

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

问题似乎是你给PL/SQL它期待直接的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它包装在一个BEGIN ... END区块中.

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,你可能会这样:

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;
/