Any introduction to the SQL programming language and database management systems comes with a discussion of querying. Querying is the method of searching through a database. SQL (Structured Querying Language) can be used to communicate with relational database systems. SQL was created to write queries: this language can be used to return information and data about a database management system and to learn more about a dataset. There are several ways that beginner data science students and professionals can use queries to work within databases.
What is Querying?
Querying refers to asking a question. Within the context of data science, querying can be used to question or request information from a database. Depending on the purpose of the project or dataset, there are many uses for querying within SQL databases. When using SQL for data science, querying is primarily used to learn more about a dataset, as well as to manage and manipulate the data within a database. Each query is also written based on a series of statements that are ordered to return information from specific parts of a database. This information is usually returned in the form of a table of values which can be analyzed by the querent i.e. the individual or entity that wrote the query and/or asked the question.
Queries are primarily used to organize or structure how information is returned and presented, making it easier to pinpoint the types of information and data that you want to know more about. Many times, queries are used to display a data table with a specific order or structure to answer a question the querent has about the dataset. If you had a database that included information about students in a class, you might write a query to order the students by name based on their GPA or even their age. In addition to these basic queries, more advanced querying can be used to work with unstructured data as well as writing subqueries.
Subqueries are a type of query which exists inside of a statement or another subquery that can be used to return information about a specific value or data table, and there are several types of subqueries that can be used. As a more advanced form of querying, sub-queries are commonly used to return information about a group within a group of data or in conjunction with another type of data. Returning to the example of data on students, a subquery can be written to discover information about data in which there are multiple categories, such as students that have a particular major and class ranking. Like a Venn diagram where certain areas overlap, subqueries allow data scientists to only return information from the overlap of multiple groups while eliminating the extraneous information which may also pertain to the groups under consideration.
Steps to Writing Queries in SQL
Although there are many types of queries that you can write with SQL, for beginners to the SQL programming language and database systems there are a few steps that you must work through before you begin the process. This includes first identifying and downloading a database management system, exploring the database and its structure, as well as understanding the structure of writing a query to search through the database.
1. Download a Database Management System
Within the realm of database management systems, there are several types of databases that you can work with to write queries depending on the type of computer or system that you have and the types of data that you are using. Database management systems can be divided between SQL and NoSQL databases, of which SQL databases are primarily used to work with structured data and NoSQL databases can be used for semi-structured or unstructured data. Some of the most commonly used SQL databases are Microsoft SQL Server, MySQL, and PostgreSQL. NoSQL databases, such as MongoDB and Cassandra, are also quite popular within the world of data science. Whether you are working with structured or unstructured data, most database management systems allow you to write queries with the SQL programming language.
2. Explore the Database Contents and Architecture
After choosing a database management system, it is important to get to know the database that you are working with and the unique components and construction of that system. While you may have your own dataset to import into a database management system, if you are working within a database for a company or one that you did not create, it is important to get to know the database. You should begin to explore the data types and metadata available to you, such as the values in the dataset, as well as the fields and tables within the database. This is especially important to the process of writing queries because you will need to know the correct names and data types within your database in order to select and return the most accurate information.
3. Common Query Statements in SQL Databases
Once you have some knowledge of your database management system and the data that it holds, you can then write queries to make requests and return the information that you need. When writing queries in SQL there are seven key statements to know, which are SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY, and LIMIT. These statements are written sequentially and each of these statements can be used to select specific types of data and to filter and aggregate data based on certain constraints. The following example includes a general query for a database that might include information about students in a school or college.
SELECT * FROM students
ORDER BY class
Based on the above example, you are asking the database to return the names of students in order of their class ranking i.e. sophomore, senior, junior, etc. When writing queries, there is also an order of operations to structuring your statements, and this order is also exemplified by the SELECT statement coming first. Noble Desktop also offers an Intro to SQL webinar which includes a step-by-step example of how to query a SQL database using these statements.
Need to learn how to Query with SQL?
Whether you are a beginner or have more advanced knowledge in working with the SQL programming language and database management systems, learning how to write different types of queries is an essential skill when working with this structured querying language. Noble Desktop’s SQL courses not only focus on specific SQL databases but how to write queries within those databases. For students that want to move from knowledge of writing basic queries to more advanced queries, the SQL Server Bootcamp includes three levels of SQL courses to build your knowledge querying databases. The curriculum for Noble Desktop’s SQL Bootcamp also focuses on introducing students to the techniques of querying and advanced querying.