Soft test pre-Test Sprint fifth Chapter basic knowledge of database system

When the 1.E-R model is transformed into a relational model, many-to-many contacts between the three entities m:n:p should be converted to a separate relational pattern, and the keyword of the relational pattern consists of three entities ' keywords.

When the E-R model is transformed into a relational model, if it is a many-to-many connection, then the connection needs to be transformed into a separate relational pattern, and the properties of the relationship pattern consist of the keywords of each entity and the properties of the contact itself, and the key (primary key) of the relational pattern consists of the keywords of each entity.

2.CD

A projection operation is a new relationship that extracts a specified attribute (column) from a relationship, an operation that operates on a column. The meaning of ∏1.3 (R) in the subject is to select the first column (a) and the third column (C) to form a new relationship from the relationship R.

3.AC

R has 4 properties, S has 2 properties and is the same as in R, all natural connections, there will be 4 columns, the record will have 3

4. Given the relationship pattern R<u,f>,u={a,b,c},f={ab→c,c→b}. The relationship R has two candidate keywords AC and ab, with 0 non-primary attributes and 3 main attributes , respectively.

The primary and non-primary properties are complementary, and a property in a relational pattern is not a primary attribute or a non-primary property. The attribute that makes up the candidate code is the primary attribute, and the other is the non-primary attribute, all of which determine whether the attribute in the relational pattern is a primary or a non-primary attribute, first requiring the solution of its candidate code.

5. If A→BC, then a→b,a→c; if a→b,a→c, then A→BC; if A→c, ab→c

Soft test pre-Test Sprint fifth Chapter basic knowledge of database system