Introduction
An SQL query can be one of the following:
- Add/Remove table
- Insert, modify, or delete rows or fields
- Get specific results
- Modify security information
General Rules of Syntax
Commands in SQL are not case sensitive.
Semicolon (;) tells the command-line SQL program
that your query is complete.
The most important elements are the keywords, such as SELECT, FROM and WHERE.
Examples will use the following table:
Name | Age | Occupation |
---|---|---|
John | 20 | Enginer |
Chris | 26 | Chef |
Bill | 22 | Student |
The Building Blocks of Data Retrieval: SELECT and FROM
Example 1
Under the relational model, data is separated into sets that resemble a table structure. This table consists of individual data elements called columns or fields. A single set of a group of fields is known as a record or row.
For instance, to create a relational database consisting of employee data, you might start with a table called EMPLOYEE that contains the following pieces of information: Name, Age, and Occupation. These three pieces of data make up the fields in
the EMPLOYEE table.
The following SQL statement retrieves all the data from the above table.
SELECT * FROM EMPLOYEE
Changing the order of the Columns
Example 2
SELECT Age, Name, Occupation FROM EMPLOYEE
will return the following table:
Age | Name | Occupation |
---|---|---|
20 | John | Enginer |
26 | Chris | Chef |
20 | BIll | Student |
Example 3
SELECT Age FROM EMPLOYEE
will return the following table:
Age |
---|
20 |
26 |
20 |
Example 4
In the previous table the Age 20 is repeated.
SELECT DISTINCT Age FROM EMPLOYEE
will return the following table:
Age |
---|
20 |
26 |