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SQL (Structured Query Language) is a computer language specifically designed to manage data stored in relational databases. It is the language used by database administrators and developers to create, read, update and delete data stored in databases. It is also used to create, modify, and query tables and other database objects.
The ability to write basic SQL queries is essential for anyone who works with data or is responsible for managing databases. It is also a great skill for anyone looking to get a job in data science, data engineering, or any other field where data is used extensively.
In this blog post, we will cover the basics of SQL query writing. We will cover how to write basic SQL queries to select, filter, join, and aggregate data. We will also look at some useful examples that demonstrate the power of SQL.
SELECT statement
The SELECT statement is the most commonly used SQL query. It allows you to select specific columns from a table or multiple tables. For example, if you wanted to select the names and ages of all the people in a table, you would use the SELECT statement.
The general syntax for the SELECT statement is:
SELECT [columns] FROM [table(s)] [WHERE [conditions]];
Let’s take a look at an example. Suppose we have a table called “people” that contains the following data:
Name Age
John 25
Mary 30
Mike 28
If we wanted to select the names and ages of all the people in the table, we would use the following SELECT statement:
SELECT Name, Age FROM people;
The result of this query would be:
Name Age
John 25
Mary 30
Mike 28
The SELECT statement is very powerful and can be used to select data from multiple tables. For example, if we had a second table called “addresses” that contained the following data:
Name Address
John 123 Main Street
Mary 234 Elm Street
Mike 345 Oak Street
We could use the following SELECT statement to retrieve both the names and addresses of all the people in the two tables:
SELECT people.Name, people.Age, addresses.Address FROM people, addresses WHERE people.Name = addresses.Name;
The result of this query would be:
Name Age Address
John 25 123 Main Street
Mary 30 234 Elm Street
Mike 28 345 Oak Street
As you can see, the SELECT statement is a very powerful query that can be used to select data from multiple tables.
FILTER statement
The FILTER statement is used to filter the results of a query by adding conditions. For example, if we wanted to select only the names and ages of people whose age is greater than 25, we would use the following FILTER statement:
SELECT Name, Age FROM people WHERE Age > 25;
The result of this query would be:
Name Age
Mary 30
Mike 28
The FILTER statement is very useful for selecting specific data from a table. It can also be used to filter data from multiple tables. For example, if we wanted to select the names and addresses of people whose age is greater than 25, we would use the following FILTER statement:
SELECT people.Name, people.Age, addresses.Address FROM people, addresses WHERE people.Name = addresses.Name AND people.Age > 25;
The result of this query would be:
Name Age Address
Mary 30 234 Elm Street
Mike 28 345 Oak Street
The FILTER statement is a very powerful query that can be used to select specific data from multiple tables.
JOIN statement
The JOIN statement is used to join two or more tables together. It is used to combine data from multiple tables into one result set. For example, if we wanted to select the names, ages, and addresses of all the people in the two tables, we would use the following JOIN statement:
SELECT people.Name, people.Age, addresses.Address FROM people JOIN addresses ON people.Name = addresses.Name;
The result of this query would be:
Name Age Address
John 25 123 Main Street
Mary 30 234 Elm Street
Mike 28 345 Oak Street
The JOIN statement is a very powerful query that can be used to select data from multiple tables.
AGGREGATE statement
The AGGREGATE statement is used to aggregate data from multiple tables. It is used to calculate aggregate values such as sums, averages, counts, and more. For example, if we wanted to calculate the average age of all the people in the two tables, we would use the following AGGREGATE statement:
SELECT AVG(people.Age) FROM people;
The result of this query would be:
AVG(people.Age) 27.33
The AGGREGATE statement is a very useful query that can be used to calculate aggregate values from multiple tables.
Conclusion
In this blog post, we have covered the basics of SQL query writing. We have looked at how to write basic SQL queries to select, filter, join, and aggregate data. We have also looked at some useful examples that demonstrate the power of SQL.
SQL is a powerful language that can be used to query and manage data stored in relational databases. It is an essential skill for anyone who works with data or is responsible for managing databases. With the knowledge and skills gained from this blog post, you will be able to write basic SQL queries to select, filter, join, and aggregate data.