Oracle 空值计算总结
CREATE TABLE T1 (ID INT ,ID1 INT);INSERT INTO T1 VALUES(1,NULL);INSERT INTO T1 VALUES(NULL,NULL);INSERT INTO T1 VALUES(2,2);COMMIT;--实际上有三条记录,包括两个字段都为空的记录admin@ORCL> SELECT * FROM T1; ID ID1---------- ---------- 1 NULL 2 2 NULL NULLadmin@ORCL> SELECT COUNT(*) FROM T1; COUNT(*)---------- 3admin@ORCL> SELECT COUNT(ID) FROM T1; COUNT(ID)---------- 2admin@ORCL> SELECT COUNT(ID1) FROM T1;COUNT(ID1)---------- 1--空值的SUM测试admin@ORCL> SELECT SUM(ID) FROM T1;--SUM值不考虑空值 SUM(ID)---------- 3admin@ORCL> SELECT AVG(ID) FROM T1;--AVG只除以了非为空的记录条数 AVG(ID)---------- 1.5admin@ORCL> SELECT SUM(ID1) FROM T1; SUM(ID1)---------- 2 admin@ORCL> SELECT AVG(ID1) FROM T1;--avg只除以了非空的记录条数 AVG(ID1)---------- 2--相加测试，NULL+NOT NULL VALUE = NULLadmin@ORCL> SELECT ID+ID1 FROM T1; ID+ID1---------- NULL 4--MAX与MIN测试，最大值和最小值也不包含空admin@ORCL> select max(ID) from t1; MAX(ID)---------- 2admin@ORCL> select min(ID) from t1; MIN(ID)---------- 1--排序测试select * from t1 order by id;admin@ORCL> select * from t1 order by id;--默认是升序,可以看出NULL 默认最大 ID ID1---------- ---------- 1 NULL 2 2 NULL NULLadmin@ORCL> select * from t1 order by id1; ID ID1---------- ---------- 2 2 NULL NULL 1 NULL--加上 NULLS first，将空值设为第一位admin@ORCL> select * from t1 order by id nulls first; ID ID1---------- ---------- 1 2 2--distinct会包含nulladmin@ORCL> select distinct id from t1; ID---------- 1 2
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