SQL is the ANSI and ISO standard for relational database management and retrieval. There are numerous database products that support SQL, including those from Oracle and Microsoft. <\/p>\n\n\n\n
Within a distributed database, the database\u2019s \u201cback end\u201d runs constantly on a server. This \u201cback end\u201d interprets data files on said server. A client computer that has a specific program allows users to call on and manipulate the data by using tables, columns, rows, and fields. <\/p>\n\n\n\n
To do this, these programs send SQL statements to the server. These statements are then interpreted by the server and result sets are returned to the client program. <\/p>\n\n\n\n
In this article, we\u2019ll be taking you through how to write SQL queries. We\u2019ll also be looking at a few examples in order to demonstrate what a SQL query looks like. It may seem daunting at first, but trust us, it becomes very simple once you get the hang of it. <\/p>\n\n\n\n
![\"How](\"https:\/\/openquery.com\/wp-content\/uploads\/2022\/02\/How-To-Write-SQL-Queries-1024x683.png\")
<\/figure><\/div>\n\n\n\nUnderstanding The Hierarchy Of A Database <\/strong><\/h2>\n\n\n\nBefore we get into the finer details of how to write SQL queries, it\u2019s essential that we set a context for this guide. <\/p>\n\n\n\n
First off, it\u2019s vital that you get accustomed to the database you\u2019re using and its structure or hierarchy. <\/p>\n\n\n\n
You might be working with a number of databases, all with different sets of data. In this instance, you\u2019ll want to zoom in on a specific location within the database, to find the data you want to work on. <\/p>\n\n\n\n
In very basic terms, a database with a hierarchical structure is a data model which sees data stored in the form of records which are organized into a tree-like structure.<\/p>\n\n\n\n
Within this tree-like structure, one main branch can have multiple branches coming off it, and the leaves represent the raw data. <\/p>\n\n\n\n
How To Write SQL Queries: Step By Step <\/strong><\/h2>\n\n\n\nTo start a SQL query, you\u2019ll need to open with a select statement. A select statement is used to query the database and highlights data that matches your criteria. <\/p>\n\n\n\n
The next step is to add all the field names you want to display. Field names are connected to particular fields which contain the data. <\/p>\n\n\n\n
After you\u2019ve inputted the field names, you need to add statement clause(s) or selection criteria. The \u201cfrom\u201d clause is required to identify the table(s) that contain the data. <\/p>\n\n\n\n
There are a number of optional clauses which can be used to narrow down your search. Whilst the \u201cfrom\u201d clause is essential, these clauses are not: <\/p>\n\n\n\n
\u201cWhere\u201d – Specifies selection conditions by which the data is called.<\/p>\n\n\n\n
\u201cOrder By\u201d – Denotes the ordering and sorting of rows of retrieved data.<\/p>\n\n\n\n
\u201cGroup By\u201d – This groups the results in sets. <\/p>\n\n\n\n
\u201cHaving\u201d – Allows a specific search condition and is used with the \u201cGroup By\u201d clause. <\/p>\n\n\n\n
With all of the above, your SQL Query should look like the following: <\/p>\n\n\n\n
Select <\/strong>[all | distinct] field 1 [field 2] > From <\/strong>table 1 [table 2] > [Where <\/strong>conditions] > [Group By <\/strong>field-list] > [Having <\/strong>conditions] > [Order By <\/strong>field-list]<\/p>\n\n\n\nHow To Write SQL Queries: An In Depth Example<\/strong><\/h2>\n\n\n\nIn this example, we\u2019ll discuss how to retrieve data from a database that holds information about customers of a large construction company. You might need to answer a set of questions regarding orders for house extensions.<\/p>\n\n\n\n
As this is a more in depth example, there are a few pieces of syntax which need to be included in the SQL query statement. Below is a blank example, with this syntax included:<\/p>\n\n\n\n
SELECT column 1, column 2 FROM table1, table 2 WHERE column2= \u2018value\u2019 ;<\/p>\n\n\n\n
As you can see, at the end of the statement, we\u2019ve included a semicolon (;). In SQL, a semicolon is a statement terminator. It\u2019s considered best practice to include a semicolon, but you\u2019ll need to include it if you\u2019re sending multiple statements.<\/p>\n\n\n\n
It\u2019s important to note that SQL isn\u2019t case-sensitive. This means that in SQL, a SELECT statement is the same as a select statement. In terms of readability, most programmers use uppercase for clauses or commands, and lowercase for everything else. <\/p>\n\n\n\n
Your boss has asked you to pull data from the server regarding all customers who had housing extensions with the last name \u201cWilliamson.\u201d<\/p>\n\n\n\n
In this instance, you would need to write a SELECT statement which will give you all the customers whose last name is Williamson. <\/p>\n\n\n\n
This statement would look something like this: <\/p>\n\n\n\n
SELECT * FROM Customers WHERE Last_Name = \u2018Williamson\u2019 ;<\/p>\n\n\n\n
If your boss then said that he needed a specific customer, but only knew the last part of the customer’s name, you would be able to find all customers with similar names by using the pattern-matching operator LIKE. <\/p>\n\n\n\n
If your boss said that the customer\u2019s first name ended in \u201cmes\u201d here is how you would find this customer. <\/p>\n\n\n\n
In the example below, the operator LIKE is represented by a % (percentage symbol). As you only know part of the customer’s first name, the LIKE operator uses the % wild card to match zero or more characters specified: <\/p>\n\n\n\n
SELECT * FROM Customer WHERE First_Name LIKE \u2018%mes%\u2019 ;<\/p>\n\n\n\n
Final Thoughts<\/strong><\/h2>\n\n\n\nWhilst the examples given in this article are very basic, you should now have a firm grasp on the fundamentals of writing SQL queries. <\/p>\n\n\n\n
There is so much more you can do with SQL, but having a strong understanding of the foundation will enable you to explore SQL more effectively. <\/p>\n\n\n\n
To start a SQL query, you\u2019ll need to open with a select statement. A select statement is used to query the database and highlights data that matches your criteria. <\/p>\n\n\n\n
The next step is to add all the field names you want to display. Field names are connected to particular fields which contain the data. <\/p>\n\n\n\n
After you\u2019ve inputted the field names, you need to add statement clause(s) or selection criteria. The \u201cfrom\u201d clause is required to identify the table(s) that contain the data. <\/p>\n\n\n\n
There are a number of optional clauses which can be used to narrow down your search. Whilst the \u201cfrom\u201d clause is essential, these clauses are not: <\/p>\n\n\n\n
\u201cWhere\u201d – Specifies selection conditions by which the data is called.<\/p>\n\n\n\n
\u201cOrder By\u201d – Denotes the ordering and sorting of rows of retrieved data.<\/p>\n\n\n\n
\u201cGroup By\u201d – This groups the results in sets. <\/p>\n\n\n\n
\u201cHaving\u201d – Allows a specific search condition and is used with the \u201cGroup By\u201d clause. <\/p>\n\n\n\n
With all of the above, your SQL Query should look like the following: <\/p>\n\n\n\n
Select <\/strong>[all | distinct] field 1 [field 2] > From <\/strong>table 1 [table 2] > [Where <\/strong>conditions] > [Group By <\/strong>field-list] > [Having <\/strong>conditions] > [Order By <\/strong>field-list]<\/p>\n\n\n\n In this example, we\u2019ll discuss how to retrieve data from a database that holds information about customers of a large construction company. You might need to answer a set of questions regarding orders for house extensions.<\/p>\n\n\n\n As this is a more in depth example, there are a few pieces of syntax which need to be included in the SQL query statement. Below is a blank example, with this syntax included:<\/p>\n\n\n\n SELECT column 1, column 2 FROM table1, table 2 WHERE column2= \u2018value\u2019 ;<\/p>\n\n\n\n As you can see, at the end of the statement, we\u2019ve included a semicolon (;). In SQL, a semicolon is a statement terminator. It\u2019s considered best practice to include a semicolon, but you\u2019ll need to include it if you\u2019re sending multiple statements.<\/p>\n\n\n\n It\u2019s important to note that SQL isn\u2019t case-sensitive. This means that in SQL, a SELECT statement is the same as a select statement. In terms of readability, most programmers use uppercase for clauses or commands, and lowercase for everything else. <\/p>\n\n\n\n Your boss has asked you to pull data from the server regarding all customers who had housing extensions with the last name \u201cWilliamson.\u201d<\/p>\n\n\n\n In this instance, you would need to write a SELECT statement which will give you all the customers whose last name is Williamson. <\/p>\n\n\n\n This statement would look something like this: <\/p>\n\n\n\n SELECT * FROM Customers WHERE Last_Name = \u2018Williamson\u2019 ;<\/p>\n\n\n\n If your boss then said that he needed a specific customer, but only knew the last part of the customer’s name, you would be able to find all customers with similar names by using the pattern-matching operator LIKE. <\/p>\n\n\n\n If your boss said that the customer\u2019s first name ended in \u201cmes\u201d here is how you would find this customer. <\/p>\n\n\n\n In the example below, the operator LIKE is represented by a % (percentage symbol). As you only know part of the customer’s first name, the LIKE operator uses the % wild card to match zero or more characters specified: <\/p>\n\n\n\n SELECT * FROM Customer WHERE First_Name LIKE \u2018%mes%\u2019 ;<\/p>\n\n\n\n Whilst the examples given in this article are very basic, you should now have a firm grasp on the fundamentals of writing SQL queries. <\/p>\n\n\n\n There is so much more you can do with SQL, but having a strong understanding of the foundation will enable you to explore SQL more effectively. <\/p>\n\n\n\nHow To Write SQL Queries: An In Depth Example<\/strong><\/h2>\n\n\n\n
Final Thoughts<\/strong><\/h2>\n\n\n\n