Recently, I am using the oracle database to select data, because I have been using mysql before, I always encounter some problems, and record it.
A select query is to query multiple tables, and then to find out whether a field in a table is empty, or not empty. If it is empty, it is unused, and if it is not empty, it is used.
Can use is null and is not null
如：
1、select * from B LEFT JOIN A on B.NO = A.NO where A.AMOUNT is null;未使用
2、select * from B LEFT JOIN A on B.NO = A.NO where A.AMOUNT is not null;已使用