Ch 9Structured Query Language (SQL) — Class 12 Computer Science, concept-first.

We have already seen that a Relational Database Management System (RDBMS) is the software that manages data organised into relations (tables).

In a file system, you have to write full application programs just to get at your data. That is a lot of work.

MySQL is an open-source RDBMS (Relational Database Management System). To use it, you first need to download it from the official website: https://dev.mysql.com/downloads.

A database is made up of tables, and each table is made up of columns (also called attributes). Every column stores a specific kind of information — for example, a student's name is text, their date o…

A database table is made up of attributes (columns), and every attribute must be of a specific data type.

Constraints are rules that restrict what data values can be stored in a column. They exist to protect the correctness and reliability of your data — for example, preventing a blank entry where a value…

Before you can store any data, you must first decide how that data will be organised. This is the core idea behind data definition.

To create a new database in MySQL, you use the CREATE DATABASE statement. The syntax is straightforward:

Once a database has been created, the next step is to define the relations (tables) that will store the data.

Before you can work with a table — insert data, query it, or modify it — you need to know its structure: what columns exist, what type of data each column holds, whether a column can be empty, and so…

After you've created a table, you'll often find you need to adjust its structure — perhaps you forgot to declare a key, need a new column, or want to tighten up the rules on an existing one.

The DROP statement is SQL's way of permanently removing a database object — either a table or the entire database itself — from the system.

Once a table is created with CREATE TABLE, only its structure exists — the table itself holds no data. To populate a table with records, we use the INSERT statement.

The INSERT INTO command is how you add new rows of data to a table. You must always specify the table name, and then provide the values you want to store. The simplest form is:

The whole point of storing data in a database is to be able to retrieve it later, in whatever way it is needed.

The SELECT statement is the most important command in SQL — it is how you retrieve data from a table. The result of a SELECT query is always displayed in tabular form, just like the original table.

Organisations keep their data in a database made up of several related tables — for example, an OFFICE database might hold an EMPLOYEE table, a DEPARTMENT table, and others besides.

Updation and deletion of data are also part of SQL's Data Manipulation Language (DML). In this section, we apply these two data manipulation methods — UPDATE and DELETE — on the STUDENT and GUARDIAN t…

The UPDATE statement is the tool for modifying existing data in a table. You use it when a value in one or more columns of a record needs to be changed — for example, correcting a spelling, updating a…

The DELETE statement removes one or more entire records (rows) from a table. It does not remove the table structure itself — only the data inside it.

A function in SQL is a pre-written block of code that performs a specific task and returns a value (or values) as a result.

Single row functions, also called scalar functions, work on a single value at a time and return a single value as output. They are applied to each row individually when used in a query.

Aggregate functions are also called multiple row functions. Unlike single row functions that work on one row at a time, these functions operate on a set of records as a whole and return a single value…

The GROUP BY clause is used when you need to work with groups of rows that share the same value in a particular column, rather than with individual rows.

Operations on relations let us combine or compare data from two tables. The textbook introduces three such operations: Union, Intersection, and Set Difference.

The UNION operation combines the selected rows from two tables into a single result set. When you apply UNION, every row that appears in either of the two tables is included in the output.

The INTERSECT operation in SQL is used to find common rows between two tables. It corresponds to the mathematical intersection of sets, represented by the symbol ∩.

The MINUS operation (also called set difference) is used to find rows that exist in the first table but not in the second table. It is represented by the symbol − (minus).

The Cartesian product (also called the cross product) is a relational algebra operation that combines every row from one table with every row from another table. It is denoted by the symbol X.

Till now, every query in this chapter has worked with data from a single relation at a time. This section introduces writing queries that use two relations together — the foundation for combining rela…

The Cartesian product is the operation that sits underneath every multi-table query in SQL. When you list more than one table in the FROM clause, separated by commas, the database engine first combine…

A database is useful because it stores related data across multiple tables. But to answer real questions, you often need to bring that data back together.

- A database is a collection of related tables; MySQL is a 'relational' DBMS. - DDL (Data Definition Language) includes SQL statements such as CREATE TABLE, ALTER TABLE, and DROP TABLE.

Questions from official CBSE sample papers.