Three basic relational operations that a relational database system can implement

The three basic relational operations that the relational database management system can implement are selection (the operation of finding tuples that meet given conditions from the relationship is called selection), projection (specifying several attributes from the relational schema to form a new relationship), connection (which is a horizontal combination of relationships).

Select

Use comparison operators and logical operators to select tuples that meet the conditions and calculate the result!

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Example

Query the names of employees whose salary is greater than 2,000

 SELECT ename FROM emp WHERE sal > 2000;

Process: First query all the results of the emp table, use the selection operation to filter out the results with a salary greater than 2000, and finally use the projection operation to get the names of employees with a salary greater than 2000!

Projection

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The operation process of projection:

Firstly, in the order of j1, j2,…,jk, the column numbers are taken out from the relationship R as j1, j2,…,jk (or the attribute name sequence is Aj1, Aj2,...,Ajk), and then remove the repeated tuples in the result to form a k-item relationship with Aj1, Aj2,...,Ajk as the attribute sequence.

To put it simply: take out a certain column in a query result and eliminate duplicate data. This is projection!

Projection is an operation from the perspective of a column

The subscript of the projection can be the column number or the column attribute name

Example

Query the numbers of all departments

SELECT deptno FROM dept;

The query process: first query to get all the results of the dept table, and then extract only the column data of "deptno" through the projection operation. If SELECT is followed by "*", then all data is projected!

Connection operation

The connection operation actually limits the conditions (a certain column is greater than, less than, or equal to a certain column) based on the Cartesian product operation, and only matches the sum If the conditions are met, the result will be obtained!

Natural connection

Natural connection is a special connection operation, and its limited condition is [a certain column is equal to a certain column]. Natural connections we use often. Eliminating the Cartesian product is actually a natural connection!

Example

SELECT * FROM emp,dept WHERE dept.deptno = emp.deptno;

Set the deptno column of the dept table to be the same as the deptno column of emp [This is a natural connection]

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