DBMS中BCNF和4NF的区别

1. 博伊斯-科德范式(BCNF)：
当且仅当满足以下条件时，才称任何关系在 BCNF 中：

• 对于每个函数依赖 (FD) X->Y，X 是给定关系中的SuperKey。

2. 第四范式(4NF)：
当满足以下条件时，任何关系都称为第四范式：

• 它必须是 Boyce Codd 范式 (BCNF)。
• 它应该没有多值依赖。

当表中有两个属性依赖于第三个属性但彼此独立时，称为多值依赖。为了表示多值依赖，使用“->->”这个符号。

BCNF 和 4NF 的区别：

S.No.	BCNF	4NF
1	A relation in BCNF must also be in 3NF.	A relation in 4NF must also be in Boyce Codd Normal Form (BCNF).
2	A relation in BCNF may have multi-valued dependency.
3	A relation in BCNF may or may not be in 4NF.	A relation in 4NF is always in BCNF.
4	BCNF is less stronger in comparison to 4NF.	4NF is more stronger in comparison to BCNF.
5	If a relation is in BCNF then it will have more redundancy as compared to 4NF.	If a relation is in 4NF then it will have less redundancy as compared to BCNF .
6	If a relation is in BCNF then all redundancy based on functional dependency has been removed.	If a relation is in 4NF then all redundancy based on functional dependency as well as multi-valued dependency has been removed.
7	For a relation, number of tables in BCNF is less than or equal to number of tables in 4NF.	For a relation, number of tables in 4NF is greater than or equal to number of tables in BCNF.
8	Dependency preserving is hard to achieve in BCNF.	Dependency preserving is more hard to achieve in 4NF as compared to BCNF.
9	In real world database designing, generally 3NF or BCNF is preferred.	In real world database designing, generally 4NF is not preferred by database designer.
10	A relation in BCNF may contain multi-valued as well as join dependency.	A relation in 4NF may only contain join dependency.