This mini-article is made for beginners. We will take a closer look on how to write a simple T-SQL script and I will also explain basic sql commands. We will put together individual parts of script piece by piece and we will understand its purpose during the process. Let’s dig into SQL basics in following example.
SQL Basics – SQL query assembly step by step
Task: Imagine table containing data on people. Task is to pick everyone in table “People” who was born between years 1995 and 2005 including. In the result of the script, we are interested in name, surname and date of birth. All records should be organized in descending order by surname.
SELECT Command
This clause states that we want do display some data. We also define in it which data (Columns) to pick:
Continuous SQL script:
SELECT
Name,
Surname,
DateOfBirth
If we would execute this script above a database it would result in error. Reason for this is that we did not declare from where fields “Name”, “Surname” and “DateOfBirth” get selected. It is needed to select the table.
Command FROM
Clause defines where are information from previous step located = in which table will we find them. We will search in table “People”
Continuous SQL script:
SELECT
Name,
Surname,
DateOfBirth
FROM People
This script will finally be functional and it will return all records from table “People”. Selected columns will be Name, Surname and DateOfBirth.
Command WHERE
It is time to restrict the result from previous step to DateOfBirth. This clause generally serves to define restricting conditions.
Continuous SQL script:
SELECT
Name,
Surname,
DateOfBirth
FROM People
WHERE DateOfBirth>= '1995-01-01' AND DateOfBirth<= '2005-12-31'
Condition consists of column we need to limit somehow. We can see that we can use mathematical operators as ><= etc. in SQL. Condition can be simplified by using operator BETWEEN.
In this case the condition would look like this: WHERE DateOfBirth BETWEEN '1995-01-01' AND '2005-12-31'.
Command ORDER BY
Last part of the task is to sort the result. We do that through command ORDER BY where we can add next to it how we want the data to be organized. (ASC – ascending order or DESC – descending order) (not necessary). If we do not fill in the sorting requirement, data will be sorted in ascending order.
Final script:
SELECT
Name,
Surname,
DateOfBirth
FROM People
WHERE DateOfBirth>= '1995-01-01' AND DateOfBirth<= '2005-12-31'
ORDER BY DateOfBirth DESC
I hope this article helped you to understand basic SQL command and SQL basics. I recommend to go through articles on this web if you prefer self-studying.
You can start by going through articles concerning SQL clauses:
- SELECT – Selection of Columns From the Table – Basic Clause
- SELECT DISTINCT – Removing Duplicates from Table = Unique values
- WHERE – Adding a condition using SQL
- GROUP BY – Aggregating Data in Table
- ORDER BY – Sorting Data in SQL Table in Descending or Ascending Order
- Difference between WHERE and HAVING – SQL HAVING vs WHERE – What’s the difference?