amirpet
Quantum Hardware Engineer
LEVEL 1
400 XP
The select command is used to retrieve data from the tables.
Consider the following table:
Users:
Firstname
Lastname
Age
Maritalstatus
Country
John
Smith
40
Married
Usa
Mathew
Simon
30
Married
UK
Bill
Steve
20
Single
Usa
Amanda
Rogers
28
Married
Germany
Steve
Hills
30
Single
france
Now to get all the data we simply query the database with the select command:
SELECT
*
FROM
Users
And the DBMS returns all set of rows in the user table.
The * in the select statement is to select all columns of the table, we can also specify certain columns that we wish to retrieve, all column titles must have to be separated by a comma.
If we want to list only the first name and last name of all users then:
SELECT
Firstname,
Lastname FROM
users
Firstname
Lastname
John
Smith
Mathew
Simon
Bill
Steve
Amanda
Rogers
Steve
Hills
We can also filter our choices and dataset on the conditions we specify.
The filtering of data set is made by the where clause, the where clause is used whenever we want some specific data that meets our selection criteria to shown.
Example: we want to list all the user that are from USA:
SELECT
*
FROM
Users WHERE
Country=
USA
Firstname
Lastname
Age
Maritalstatus
Country
John
Smith
40
Married
Usa
Bill
Steve
20
Single
Usa
Similarly we can also apply our data by filtering on numeric values. For example we want to get all the users who are above or equal to 30 years of age:
SELECT
*
FROM
Users WHERE
Age >=
30
Firstname
Lastname
Age
Maritalstatus
Country
John
Smith
40
Married
Usa
Mathew
Simon
30
Married
UK
Steve
Hills
30
Single
france
We can also specify multiple conditions in the where clause.
For example if we want to find the users living in USA and are married:
SELECT
*
FROM
Users WHERE
county=
’USA’ AND
Maritalstatus=
’Married’
Firstname
Lastname
Age
Maritalstatus
Country
John
Smith
40
Married
Usa
Consider the following table:
Users:
Firstname
Lastname
Age
Maritalstatus
Country
John
Smith
40
Married
Usa
Mathew
Simon
30
Married
UK
Bill
Steve
20
Single
Usa
Amanda
Rogers
28
Married
Germany
Steve
Hills
30
Single
france
Now to get all the data we simply query the database with the select command:
SELECT
*
FROM
Users
And the DBMS returns all set of rows in the user table.
The * in the select statement is to select all columns of the table, we can also specify certain columns that we wish to retrieve, all column titles must have to be separated by a comma.
If we want to list only the first name and last name of all users then:
SELECT
Firstname,
Lastname FROM
users
Firstname
Lastname
John
Smith
Mathew
Simon
Bill
Steve
Amanda
Rogers
Steve
Hills
We can also filter our choices and dataset on the conditions we specify.
The filtering of data set is made by the where clause, the where clause is used whenever we want some specific data that meets our selection criteria to shown.
Example: we want to list all the user that are from USA:
SELECT
*
FROM
Users WHERE
Country=
USA
Firstname
Lastname
Age
Maritalstatus
Country
John
Smith
40
Married
Usa
Bill
Steve
20
Single
Usa
Similarly we can also apply our data by filtering on numeric values. For example we want to get all the users who are above or equal to 30 years of age:
SELECT
*
FROM
Users WHERE
Age >=
30
Firstname
Lastname
Age
Maritalstatus
Country
John
Smith
40
Married
Usa
Mathew
Simon
30
Married
UK
Steve
Hills
30
Single
france
We can also specify multiple conditions in the where clause.
For example if we want to find the users living in USA and are married:
SELECT
*
FROM
Users WHERE
county=
’USA’ AND
Maritalstatus=
’Married’
Firstname
Lastname
Age
Maritalstatus
Country
John
Smith
40
Married
Usa