SQL (Structured Query Language) is a programming language used to manage and manipulate relational databases. It is the standard language used for querying and managing data in most modern databases. In this article, we will discuss the basics of SQL, including its history, syntax, and usage.
History of SQL SQL was first developed in the 1970s by IBM researchers Donald Chamberlin and Raymond Boyce. Originally called SEQUEL (Structured English Query Language), it was designed to make it easier for non-technical users to access and manipulate data stored in relational databases. It was later renamed SQL when it became an ANSI (American National Standards Institute) standard in 1986.
Since then, SQL has become the standard language used in most modern databases, including Oracle, Microsoft SQL Server, MySQL, and PostgreSQL. It has undergone several revisions and updates over the years, with the latest version being SQL:2019.
SQL Syntax SQL is a declarative language, which means that you specify what you want the database to do, rather than how to do it. Here are some of the basic syntax rules of SQL:
SQL statements are typically written on separate lines, although some statements can be written on a single line.
SQL keywords are not case sensitive, but it is a common convention to write them in uppercase letters.
SQL statements are terminated by a semicolon (;).
SQL comments can be added to a statement using the — symbol.
Here is an example of a basic SQL statement:
SELECT column1, column2, ... FROM table_name WHERE condition;
This statement selects the specified columns from the specified table, based on the specified condition.
SQL Data Types SQL supports a wide range of data types, including numeric, character, and date/time data types. Here are some of the most commonly used data types in SQL:
Numeric data types: INTEGER, SMALLINT, BIGINT, DECIMAL, FLOAT, and REAL.
Character data types: CHAR, VARCHAR, and TEXT.
Date/time data types: DATE, TIME, TIMESTAMP, and INTERVAL.
SQL Operators SQL supports a wide range of operators, including arithmetic operators, comparison operators, and logical operators. Here are some of the most commonly used operators in SQL:
Arithmetic operators: +, -, *, /, and %.
Comparison operators: =, <>, >, >=, <, and <=.
Logical operators: AND, OR, and NOT.
SQL Statements SQL statements are used to manipulate and manage data in a database. Here are some of the most commonly used SQL statements:
SELECT: Retrieves data from one or more tables.
INSERT: Adds new data to a table.
UPDATE: Modifies existing data in a table.
DELETE: Removes data from a table.
CREATE: Creates a new table, view, or index.
ALTER: Modifies the structure of an existing table, view, or index.
DROP: Deletes an existing table, view, or index.
GRANT: Grants permissions to a user or group to access data in a database.
REVOKE: Removes permissions to access data in a database.
SQL Joins SQL joins are used to combine data from two or more tables based on a common column. Here are some of the most commonly used SQL joins:
INNER JOIN: Returns only the rows that have matching values in both tables.
LEFT JOIN: Returns all the rows from the left table and the matching rows from the right table.
RIGHT JOIN: Returns all the rows from the right table and the matching rows from the left table.
FULL OUTER JOIN: Returns all the rows from both tables, including the unmatched rows.
SQL Functions SQL functions are used to perform calculations or manipulate data in a database. Here are some of the most commonly used SQL functions:
Aggregate functions: SUM, AVG, COUNT, MIN, and MAX.
String functions: CONCAT, LENGTH, UPPER, LOWER, and SUBSTR.
Date/time functions: DATEADD, DATEDIFF, DATEPART, and GETDATE.
Mathematical functions: ABS, CEILING, FLOOR, ROUND, and SIGN.
SQL Constraints SQL constraints are used to specify rules that must be enforced when data is added, modified, or deleted from a database. Here are some of the most commonly used SQL constraints:
NOT NULL: Ensures that a column cannot contain null values.
UNIQUE: Ensures that each value in a column is unique.
PRIMARY KEY: Combines the NOT NULL and UNIQUE constraints to create a unique identifier for each row in a table.
FOREIGN KEY: Establishes a relationship between two tables, based on a common column.
CHECK: Ensures that the values in a column meet a specified condition.
SQL Indexes SQL indexes are used to improve the performance of database queries by allowing the database to quickly locate the data it needs. Here are some of the most commonly used SQL indexes:
Clustered index: Organizes the data in a table based on the values in one or more columns.
Non-clustered index: Creates a separate index that allows the database to quickly locate the data it needs.
Unique index: Ensures that each value in a column is unique, and creates an index to improve performance.
Full-text index: Allows users to search for text within a column.
Conclusion SQL is a powerful and versatile language used to manage and manipulate data in relational databases. Its simple syntax, wide range of data types and operators, and rich set of statements, functions, constraints, and indexes make it easy to use and highly effective for managing large amounts of data. Whether you are a database administrator, developer, or data analyst, SQL is a language you need to know in order to succeed in the field of data management.