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SELECT Statement and its Clauses in DBMS
The select statement is used to get the required data from the database according to the conditions, if any. This data is returned in the form of a table.
The basic syntax of the select statement is −
Select column 1, column 2 ... column N From table_name
An example of the select statement is −
<Student>
Student_Number | Student_Name | Student_Phone | Student_Marks | Student_Major Subject |
1 | Andrew | 6615927284 | 95 | Literature |
2 | Sara | 6583654865 | 65 | Maths |
3 | Harry | 4647567463 | 48 | Literature |
4 | Sally | 6537837084 | 30 | Literature |
5 | Anne | 7457337732 | 88 | Maths |
Query −
Select Student_Name From Student
This query yields the following result −
Student_Name |
Andrew |
Sara |
Harry |
Sally |
Anne |
Clauses in Select statement
The example of select statement given above is quite simple and not that useful in practice. So, there are many other clauses associated with select statement that make it more meaningful. Some of these are −
Where
The where clause is used to filter out data i.e it returns information that satisfies a certain condition. For example −
Select Student_Name From Student Where Student_Marks >50
This query will return the following result:
Student_Name |
Andrew |
Sara |
Anne |
Group by
This is mostly used with aggregate functions to group the result set according to the value of a column. For example −
Select Count (Student_Number), Student_MajorSubject From Student Group by Student_MajorSubject
This query will return the following result −
Count (Student_number) | Student_MajorSubject |
3 | Literature |
2 | Maths |
Having
This is used along with Group By clause because Where clause could not be used by aggregate functions. For Example −
Select Count(Student_number), Student_MajorSubject From Student Group by Student_MajorSubject Having Count(Student_Number) > 2
This query will return the following result −
Count (Student_Number) | Student_MajorSubject |
3 | Literature |
Order by
The order by keyword is used to sort the results in ascending or descending order. By default, the order is assumed to be ascending. For Example −
Select Student_Name From Student Where Student_Marks>50 Order by Student_Marks
This query will return the following result −
Student_Name |
Sara |
Anne |
Andrew |