SQL (Structured Query Language) is a standard language used to manage and manipulate relational databases. Whether you’re starting your journey as a database administrator or looking to improve your SQL skills, this beginner’s guide from Novatesting to SQL commands is for you.
SQL Commands: Data Definition Language Commands (DDL)
DDL (Data Definition Language) commands are used to create, modify and delete database structures, including tables, indexes, and constraints. Some common DDL commands include:
- CREATE TABLE: creates a new table in the database
- ALTER TABLE: modifies an existing table structure
- DROP TABLE: deletes an existing table
- TRUNCATE TABLE: removes all data from a table
- RENAME TABLE: changes the name of a table
SQL Commands: Different Types Of Keys In Database
A key is a database column that is used to uniquely identify a record in a table. There are several types of keys in databases, including:
- Primary Key: a unique identifier for each record in a table. There can only be one primary key per table.
- Foreign Key: links one table to another, creating a relationship between the two tables.
- Composite Key: a combination of two or more columns used as a primary key in a table.
- Candidate Key: a set of columns that can be used as a primary key in a table.
SQL Commands: Constraints Used In Database
Constraints are used to enforce rules in a database and maintain data integrity. Some common constraints include:
- NOT NULL: ensures that a column cannot contain a null value
- UNIQUE: ensures that a column contains unique values
- PRIMARY KEY: enforces the primary key constraint for a column or set of columns
- FOREIGN KEY: enforces the foreign key constraint for a column or set of columns
SQL Commands: Data Manipulation Language Commands (DML)
DML (Data Manipulation Language) commands are used to insert, update, and delete data in a database. Some common DML commands include:
- SELECT: retrieves data from a database
- INSERT: inserts new data into a database
- UPDATE: updates existing data in a database
- DELETE: deletes data from a database
SQL Commands: Nested Queries
Nested queries are SQL commands that are executed within another SQL command. This allows you to use the results of one query to define the conditions of another query. Nested queries are often used to retrieve data from multiple tables.
SQL Commands: Joins
Joins allow you to combine data from two or more tables in a database. There are several types of joins, including:
- INNER JOIN: returns only the rows that have matching values in both tables
- LEFT JOIN: returns all rows from the left table and the matching rows from the right table
- RIGHT JOIN: returns all rows from the right table and the matching rows from the left table
- FULL OUTER JOIN: returns all rows from both tables, regardless of whether there is a matching value in the other table
SQL Commands: Set Operations
Set operations allow you to combine the results of two or more queries into a single result set. Some common set operations include:
- UNION: combines the results of two or more queries and returns only the unique values
- INTERSECT: returns the common values from two or more queries
- MINUS: returns the values from the first query that are not in the second query
SQL Commands: Dates & Auto Increment
Dates and auto increment are two important features in SQL databases. Dates allow you to store and manipulate date and time values, while auto increment allows you to automatically generate unique values for a column.
SQL Commands: Data Control Language Commands (DCL)
DCL (Data Control Language) commands are used to manage database security and permissions. Some common DCL commands include:
- GRANT: grants specific privileges to a user or role
- REVOKE: revokes specific privileges from a user or role
SQL Commands: Views
Views are virtual tables that allow you to simplify the structure of a database by presenting data from multiple tables in a single result set. Views are often used to restrict access to certain data or to simplify complex queries.
SQL Commands: Stored Procedures
Stored procedures are pre-written SQL commands that can be executed with a single call. Stored procedures are often used to automate repetitive tasks or to enforce business rules in a database.
SQL Commands: Triggers
Triggers are SQL commands that are automatically executed in response to specific events in a database. Triggers are often used to enforce business rules or to automate complex processes in a database.
SQL Commands: Transaction Control Language Commands (TCL)
TCL (Transaction Control Language) commands are used to manage transactions in a database. Some common TCL commands include:
- COMMIT: saves changes to a database
- ROLLBACK: reverses changes to a database
- SAVEPOINT: creates a checkpoint in a transaction
- SET TRANSACTION: sets transaction properties, such as isolation level and transaction name
In conclusion, SQL commands are an essential tool for managing and manipulating relational databases. Whether you’re just starting out or looking to expand your skills, this beginner’s guide to SQL commands is a great place to start. By mastering these commands, you’ll be able to work with databases with greater ease and efficiency.