Soft test --- database (3) data operation

Data manipulation

1.1 Common relational operations

Inquire about ：

choice 、 Projection 、 Connect 、 except 、 and 、 hand over 、 Bad

Data update :

• Insert
• Delete
• modify

The expression ability of query is the most important part

• The traditional set operation is binomial operation , Include and 、 hand over 、 Bad 、 The generalized Cartesian product ;
• Specialized relational operations include ： choice 、 Projection 、 Connect 、 except 4 Operations ;

1.1.1 Set operation noun interpretation

and ：

R and S Is not owned by R And belong to S A collection of tuples , Write it down as R ∪ S ;

Bad ：

R and S The difference is that it will only belong to R A collection of tuples , Write it down as R - S ;

hand over ：

R and S The intersection of belongs to R and S A collection of tuples , Write it down as R ∩ S ;

1.2 Relationship between operation

Relational databases also have some special operations , Mainly projection 、 choice 、 Division and outer connection ;

In Relational Algebra , The formula which is composed by five basic algebraic operations through finite times is called relational algebraic operation expression , The operation result of the expression is still a relational expression ;

1.2.1 Projection

Projection is to map some attributes or selected columns in a relationship , And remove the tuples with duplicate attributes ;

1.2.2 choice

The selection operation is to select all tuples that meet the given conditions in the relationship ;

1.2.3 Connect

Connection is to obtain tuples satisfying certain conditions from the Cartesian product of two relations ;

1.2.4 division

Set up a relationship R Divide by relation S The result is relationship T , be T Include all in R But not here. S Properties and their values in , And T The tuple of and S All combinations of tuples of are in R in ;