Query is a question or requesting information. Query language is a language which is used to retrieve information from a database.
Query language is divided into two types −
Information is retrieved from the database by specifying the sequence of operations to be performed.
For Example − Relational algebra.
Structure Query language (SQL) is based on relational algebra.
Relational algebra consists of a set of operations that take one or two relations as an input and produces a new relation as output.
The different types of relational algebra operations are as follows −
Cartesian product operation
Select, project, rename comes under unary operation (operate on one table).
It displays the records that satisfy a condition. It is denoted by sigma (σ) and is a horizontal subset of the original relation.
Its syntax is as follows −
Consider the student table given below −
Now, to display all the records of student table, we will use the following command −
In addition to this, when we have to display all the records of CSE branch in student table, we will use the following command −
Hence, the result will be as follows −
To display all the records in student tables whose regno>2, we will use the below mentioned command −
The output will be as follows −
To display the record of ECE branch section B students, use the given command −
σbranch=ECE ^ section=B(student)
To display the records of section B CSE and IT branch, use the following command −
σSection=B ^ Branch=cse ∨ branch=IT(student)
Consider the EMPLOYEE TABLE as another example to know about selection operations.
Retrieve information about those employees whose salary is greater than 20,000
If one condition is specified then, we can use the following command −
σ salary > 20,000 (emp)
If more than one condition specified in the query then ( AND: ^, OR:∨ , Not:#, equal: =, >, <, >=, <=)
Relational operator will be used to combine the multiple conditions into a single statement.
Example − In order to retrieve the information of those employee whose salary > 20,000 and working in HOD and Dept no is 20, we can use the following command −
σ salary > 20,000 ^LOC=HOD ^Deptno=20(emp)