1、概念
某一列唯一键的数量叫做基数。
2、举例
性别列，该列只分男女，so 这一列的基数为2。
3、测试
SYS@orcl> select count(distinct owner),count(distinct object_id),count(*) from t;
COUNT(DISTINCTOWNER) COUNT(DISTINCTOBJECT_ID) COUNT(*)
-------------------- ------------------------ ----------
20 83962 83962
t表的总行数为83962，owner列的基数为20，object_id的基数83962，说明owner列有大量的重复值。object_id很有可能是主键。
SYS@orcl> select owner,count(*) from t group by owner order by 2 desc;
OWNER COUNT(*)
------------------------------ ----------
SYS 37365
PUBLIC 33339
SYSMAN 3554
APEX_030200 2561
ORDSYS 2513
MDSYS 1447
XDB 1167
SYSTEM 611
CTXSYS 389
WMSYS 333
EXFSYS 312
ORDDATA 257
DBSNMP 57
FLOWS_FILES 13
OUTLN 10
ORDPLUGINS 10
ORACLE_OCM 8
SI_INFORMTN_SCHEMA 8
APPQOSSYS 5
ROIDBA 3
20 rows selected.
SYS@orcl>
原文：http://blog.51cto.com/roidba/2323119