Views Views are virtual tables derived from SQL queries. They don't store data physically but present filtered perspectives of base tibles. -- Create sample view for male students CREATE VIEW male_students AS SELECT * FROM students WHERE gender = 'male'; -- Join view with classes table CREATE VIEW m...
SQL provides a robust set of constructs for retrieving, filtering, transforming, and modifying relational data. Below are foundational patterns used across database systems—with syntax adapted for portability and clarity. 1. Basic Column Selection Retrieve specific columns instead of all fields to i...
The problem requires calculating the average selling price for each product, where the average selling price equals the total revenue divided by the quantity sold. Solution approach: We need to compute the total revenue and total quantity for each product. The total quantity can be obtained using th...
Extracting JSON Values in MySQL MySQL provides several built-in methods to extract and manipulate JSON data. These techniques vary based on whether you need raw JSON values, unquoted strings, or support for dynamic paths. Basic Field Access The -> operator retrieves a value from a JSON document w...
When performing joins in SQL where the primary table is very large, query performence can degrade significant. A common optimization technique is to reduce the dataset size early by appyling filters in a subquery before joining. This minimizes the number of rows involved in the join operation and of...
1. Recyclable and Low-Fat Products Retreive product IDs where both low_fats and recyclable are 'Y'. SELECT product_id FROM Products WHERE low_fats = 'Y' AND recyclable = 'Y'; 2. Find Customer Referees Select customers whose referee is not ID 2, including those with no referee (NULL). SELECT name FRO...
Overview of JDBC Architecture Java Database Connectivity (JDBC) is a standard Java API that defines how a client may access a database. It provides a set of interfaces and classes written in Java to execute SQL statements. Essentially, JDBC acts as a middle tier between Java applications and a wide...
Introduction to Data Query Language Data Query Language (DQL) is a subset of SQL (Structured Query Language) specifically designed for retrieving data from databases. Common DQL statements and their functions include: SELECT: Retrieves data from a database. Example: SELECT column1, column2 FROM myt...
Inner joins filter result sets by matching keys across related tables, returning only rows where the join predicate evaluates to true in both sources. Syntax Variations Explicit join notation uses the INNER JOIN keyword with an ON clause: SELECT target_columns FROM primary_table INNER JOIN secondary...
Session-Level Cache Every SqlSession holds an internal cache that is active by default. This local storage avoids redundant database calls when identical queries run inside the same session. Internal Mechanics The cache is backed by a Map scoped to the session. When a query arrives, MyBatis checks t...