SQL, or Structured Query Language, is a programming language used to manage and manipulate relational databases. Advanced SQL is a set of techniques and features that allow developers to perform more complex and efficient operations on databases. In this chapter, we'll explore some of these advanced features of SQL.
Subqueries
Subqueries, also known as nested queries, are SQL queries that are embedded inside other SQL queries. They are used when the operation you want to perform is too complex to be performed with a single SQL query. A subquery can return one or more values and can be used in many parts of a query, including the SELECT, FROM, WHERE, and HAVING clauses.
Joins
SQL joins are used to combine rows from two or more tables, based on a related column between them. There are four main types of SQL joins: INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN. INNER JOIN only returns rows where there is a match in both tables. LEFT JOIN returns all rows from the left table and matching rows from the right table. RIGHT JOIN returns all rows from the right table and matching rows from the left table. FULL JOIN returns all rows when there is a match in one of the tables.
Aggregation functions
SQL aggregate functions are used to perform calculations on a set of values and return a single value. The most common aggregate functions include COUNT, SUM, AVG, MAX, and MIN. The COUNT function returns the number of rows that match a specified criteria. The SUM function returns the sum total of a numeric column. The AVG function returns the average of the values in a numeric column. The MAX and MIN functions return the maximum and minimum value in a numeric column, respectively.
Indices
SQL indexes are used to retrieve data from a database more quickly. They are similar to indexes in a book and allow the database to find data without having to search every row in the table. Indexes can be created on one or more columns of a table and can be used to speed up query operations on the database.
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Triggers
SQL triggers are stored procedures that are executed automatically in response to specific events in a database. They are used to maintain data integrity and can be triggered by events such as INSERT, UPDATE, and DELETE. A trigger can be defined to run before or after the event that triggers it.
Stored Procedures
Stored procedures are a set of SQL statements that can be stored in a database and executed as a single unit. They are used to encapsulate complex business logic, improve performance and ensure data security.
In summary, Advanced SQL provides a number of powerful features that allow developers to work with databases more efficiently and effectively. With the knowledge of these advanced techniques, you will be able to manipulate and manage data in more complex ways, helping to meet the most sophisticated business needs.
