SQL Query Data (SELECT Statement)

In this tutorial, you will learn how to use SQL to select records from a database table.

Selecting data from the table

In the previous chapter, we learned how to insert data into a database table. Now it's time to use SQL queries to select data from existing tables.

The SELECT statement is used to select or retrieve data from one or more tables. You can use this statement to retrieve all rows in a table at one time, as well as retrieve those rows that meet specific conditions or combinations of conditions.

Syntax

The basic syntax for selecting data from a table can be given as follows:

SELECT　column1_name,　column2_name,　columnN_name　FROM　table_name;

Here,column1_name，column2_name，...is the name of the database table column or field you want to retrieve its value. However, if you want to retrieve the values of all available columns in the table, you can use the following syntax:

SELECT　*　FROM　table_name;

Let's put these statements into practical use. Suppose we have a database namedemployeesThe table, which contains the following records:

+--------+--------------+------------+--------+---------+
|　emp_id　|　emp_name　　　　　|　hire_date　　|　salary　|　dept_id　|
+--------+--------------+------------+--------+---------+
|　　　　　　1　|　Ethan　Hunt　　　|　2001-05-01　|　　　5000　|　　　　　　　4　|
|　　　　　　2　|　Tony　Montana　|　2002-07-15　|　　　6500　|　　　　　　　1　|
|　　　　　　3　|　Sarah　Connor　|　2005-10-18　|　　　8000　|　　　　　　　5　|
|　　　　　　4　|　Rick　Deckard　|　2007-01-03　|　　　7200　|　　　　　　　3　|
|　　　　　　5　|　Martin　Blank　|　2008-06-24　|　　　5600　|　　　　NULL　|
+--------+--------------+------------+--------+---------+

Query and display all data in the table

The following statement will returnemployeesAll rows in the table.

SELECT　*　FROM　employees;

After execution, the output will be as follows:

+--------+--------------+------------+--------+---------+
|　emp_id　|　emp_name　　　　　|　hire_date　　|　salary　|　dept_id　|
+--------+--------------+------------+--------+---------+
|　　　　　　1　|　Ethan　Hunt　　　|　2001-05-01　|　　　5000　|　　　　　　　4　|
|　　　　　　2　|　Tony　Montana　|　2002-07-15　|　　　6500　|　　　　　　　1　|
|　　　　　　3　|　Sarah　Connor　|　2005-10-18　|　　　8000　|　　　　　　　5　|
|　　　　　　4　|　Rick　Deckard　|　2007-01-03　|　　　7200　|　　　　　　　3　|
|　　　　　　5　|　Martin　Blank　|　2008-06-24　|　　　5600　|　　　　NULL　|
+--------+--------------+------------+--------+---------+

As you can see, it returnsemployeesAll rows and columns in the table.

Select the specified columns from the table

If you do not need all the data, you can select specific columns as shown below:

SELECT　emp_id,　emp_name,　hire_date,　salary
FROM　employees;

After executing the above statement, you will get the following output:

+--------+--------------+------------+--------+
|　emp_id　|　emp_name　　　　　|　hire_date　　|　salary　|
+--------+--------------+------------+--------+
|　　　　　　1　|　Ethan　Hunt　　　|　1995-10-30　|　　　5000　|
|　　　　　　2　|　Tony　Montana　|　1990-07-15　|　　　6500　|
|　　　　　　3　|　Sarah　Connor　|　2011-04-13　|　　　5600　|
|　　　　　　4　|　Rick　Deckard　|　2005-10-18　|　　　7200　|
|　　　　　　5　|　Martin　Blank　|　1996-05-24　|　　　8000　|
+--------+--------------+------------+--------+

As you can see, this time there is nodept_idIn the next chapter, we will learn how to select records from a table based on conditions.