Q: 40 Which two statements are true about constraints? (Choose two.)
A. The UNIQUE constraint does not permit a null value for the column.
B. A UNIQUE index gets created for columns with PRIMARY KEY and UNIQUE constraints.
C. The PRIMARY KEY and FOREIGN KEY constraints create a UNIQUE index.
D. The NOT NULL constraint ensures that null values are not permitted for the column.
Answer: B, D
问下，这里A为什么不对呢，unique约束不允许null值列也是正确的啊。如何不选呢？
------解决方案--------------------------------------------------------
unique允许null列阿
SQL> alter table test add name varchar2(20) unique;
Table altered.
SQL> select * from test;
ID NAME
---------- --------------------
1.2.1
1.1
1.2
1.10
1.12
1.1.2
1.1.1
1.1.12
222
9 rows selected.
------解决方案--------------------------------------------------------
A. The UNIQUE constraint does not permit a null value for the column.
唯一约束列是允许有空值的,但主键列不能为空;这是唯一约束和主键约束的区别
B. A UNIQUE index gets created for columns with PRIMARY KEY and UNIQUE constraints.
C. The PRIMARY KEY and FOREIGN KEY constraints create a UNIQUE index.
主键和外键并没有约束创建唯一索引
D. The NOT NULL constraint ensures that null values are not permitted for the column.
所以是B和D