SQL tutorial helps you learn how to work with data through the use of structured query language. This is an important skill for software developers, database administrators, and data scientists. This SQL article provides knowledge about grouping data in a result set using GROUP BY clause. It also considers logical operators and how to apply them in the WHERE clause.
Basics
SQL is a programming language that helps you manage data in relational database management systems. It’s used to query, insert, update, and delete data from tables and view. It can also be deployed as a data control language (DCL), to protect a database from unauthorized users. Codecademy’s SQL tutorial covers the basic concepts of this popular programming language, including its syntax, and various commands. You’ll also learn how to create and manipulate database tables and views.
This tutorial will also teach you how to use logical operators, such as AND, BETWEEN, LIKE, and NOT to form a Boolean expression in the WHERE clause. You’ll also get a good understanding of how to combine multiple conditions using the OR operator. Moreover, you’ll be introduced to the concepts of SQL aliases.
Structured Query Language
SQL (pronounced ess-kew-ell) is the primary vehicle for querying and manipulating data in relational databases. Originally developed by IBM in the 1970s, SQL is now used by most major relational database management systems such as Oracle, Microsoft SQL Server, MySQL, and others.
The basic structure of an SQL statement is simple: a command or clause followed by identifiers and, optionally, a search condition. The SELECT command, for example, retrieves rows from the table based on one or more identifiers, and you can specify a limiting condition by adding a WHERE clause. Each statement must be terminated with a semicolon. In addition, every identifier must be enclosed in square brackets and all clauses are capitalized. The SET command can be used to set the value of a column or row.
Querying Data
SQL lets you query data from tables. Using the SELECT command, you can retrieve specific columns or attributes from tables. The SELECT statement also allows you to filter or sort your data. For example, you can search the ‘films’ table for movies by their profits (‘gross’ – ‘budget’). You can also perform simple arithmetic with SELECT. For example, you can calculate the average movie profit in the ‘films’ table by dividing the total gross revenue by the total number of tickets sold.
This free SQL tutorial introduces you to the basic concepts of the Structured Query Language. It includes a primer on how tables are organized in databases and the most common SQL commands like SELECT, INSERT, UPDATE, DELETE. The tutorial also covers advanced topics, such as modifying table structures, aggregating and grouping data together, creating views to simplify and secure table access and the concept of correlated subqueries.
Creating Tables
Tables are the basic building blocks of databases. They provide a structured reference for data, making it easier to read and process. When creating a table, you must name it and define its columns. The column definition specifies the names of the columns and their data types. The data types specify the kind of data the column stores (for example, varchar for strings up to 255 characters).
You also must add column constraints. These are the rules that data stored in a column must follow, such as not null and primary key. You can also add table constraints such as foreign key relationships and check constraints. You can add these constraints with the SQL CREATE TABLE statement. You can also use the Table Design tab and Table Tools group to modify the appearance of a table.
Creating Views
A view is a virtual table that logically represents data from one or more base tables. To create a view, you use the CREATE VIEW statement. You specify a name for the view and then the SELECT query whose results you want to save as a view. For example, you might create a view that displays data from the dogs and employees tables and includes columns for each dog’s owner, date of birth, and vaccination status. You might also add a foreign key constraint to ensure that values entered in the walker column of the dogs table correspond to employees’ emp_ID records.
Although views are convenient, they can have limitations. For instance, they depend on the structure of base tables and may require extra time to compute.
Conclusion
SQL is a standard language used to operate relational databases. It is widely used by many database vendors, who each have their own proprietary extensions. Use the SELECT command to retrieve data from a table based on criteria you specify. Learn about SQL WHERE clauses to filter data with multiple conditions at once. Use SQL aggregate functions to calculate summaries across multiple columns.