Indexing in SQL
Introduction Finding a few records in a large table can require substantial work if the database must examine every row. An index provides an additional access path that can help locate the require...
Query Lifecycle
Introduction A SQL query describes the information you want, but it does not usually specify how the database should retrieve it. The database must interpret the statement, choose a suitable execut...
NULL vs 0 vs Empty String
Introduction A missing value, a numeric zero, and a blank text value can look similar in a query result. However, they represent different information and can produce different results in calculati...
DDL vs DML
Introduction Working with a database involves defining how information is organized and managing the records stored within that structure. Creating a student table and updating a student’s marks...
Constraints in SQL
Introduction Choosing a data type defines what kind of information a column stores, but it does not express every requirement. An integer column, for example, can still accept a negative age unless...
Foreign Key
Introduction Related information is often stored in separate tables. A college database might keep student details in one table and course enrollments in another, with a student identifier connecti...
Primary Key
Introduction A table needs a reliable way to distinguish one record from another. Names, departments, and other descriptive values can repeat, so they may not identify a particular row accurately....
Creating and Managing Tables in SQL
Introduction Tables organize related information into rows and columns. A college database, for example, can use a students table to store student identifiers, names, and departments in a consisten...
SQL Data Types
Introduction A table can store names, prices, dates, and many other kinds of information. Each column needs a suitable data type so the database can interpret its values and apply the appropriate o...
Working with Databases in SQL
Introduction Before creating tables or writing queries, you need a database in which to organize your data. A college application, for example, might use one database to hold information about stud...
SQL Basics and Commands
Introduction Relational databases organize data into tables, but storing information is only part of the work. Applications also need to retrieve selected records, add new entries, update existing...
OS Protection and Security
Introduction An operating system is not only a resource manager. It is also a safety boundary that controls what applications are allowed to do. Protection is needed because a system contains priva...