假设我有一个这样的查询(不太实际，我知道这不是方法)
SELECT *
FROM CO.CUSTOMERS
WHERE CUSTOMER_ID = '1'
AND length((SELECT FULL_NAME
FROM CO.CUSTOMERS
WHERE CUSTOMER_ID=1)) > '35';
-- in the last as you can see I'm comparing it with a string ( i think )
但上面的查询仍然给出了结果，所以我想知道是与字符串进行比较，还是与字符串组成的整数进行比较.
在这种情况下，Oracle执行implicit datatype conversion.如果它成功做到了这一点，它将返回结果.否则，它将失败.
例如，您可以比较length > 'A2'；因为A2(字符串)不能转换为数值，所以您会得到一个错误.
SQL> select count(*) from dual where length(dummy) > '-25';
COUNT(*)
----------
1
SQL> select count(*) from dual where length(dummy) > 'A2';
select count(*) from dual where length(dummy) > 'A2'
*
ERROR at line 1:
ORA-01722: invalid number
SQL>