SQL is a crucial skill for programmers and developers alike in today’s world. It has been widely adopted as the standard language for communicating with data stored in relational databases, based on the relational model that implements SQL.
SQL or Structured Query Language is there for over a decade. Collection of tables storing specific structural data termed as “SQL Databases.” Typically, SQL deals with relational databases. Databases are crucial components of today’s/modern web development. Every web application needs a database. Without it, a web application cannot function.
Even if you know SQL well, make sure to learn or revise the following topics. Be it data science, analyst interviews, or website development; these SQL concepts are essential. Being the heart of an application, the study of SQL database needs to be thorough. Let’s get started with each concept.
1) SQL is Relational Database:
Database systems can be hierarchical, relational, document, etc. And SQL deals with the relational database.
RDBMS is the foundation of SQL and all other modern database systems. It stands for Relational Database Management System. The data is stored in RDBMS in the form of tables made up of rows and columns.
A database is a cluster of organized data that is effectively available and accessible anytime. The relational database is a set of data containing predefined relationships between them in the form of tables.
The table is also known as “Relation,” which can have multiple categories of data. Tables, keys, attributes, records are in SQL, a column is an attribute, and a row is a tuple or record. In keys, a primary key is in each table, whereas a foreign key links the primary keys of different tables. Tasks such as creation, maintenance, and retrieval of data stored in RDBMS are SQL’s jobs.
2) Keys in SQL:
A Combination of multiple fields or a single field in a table is known as Key. A key helps the user to fetch and retrieve data from the table according to specific requirements. You can use keys for various activities, not limiting to just retrieval of data.
There are seven types of SQL keys:
- Primary Key: Attribute that can uniquely identify all the attribute values in a given row and cannot be null or duplicate. Each candidate key can become a primary key, but only one can be a primary key.
- Candidate Key: The candidate key is similar to the primary key. A primary key must be unique, cannot be null or duplicate. Whereas a candidate key identifies a record uniquely in a table, there can be multiple candidate keys.
- Super Key: A key or group of keys used to identify a record/data uniquely in a table. The Keys like a primary, candidate and alternate are subsets of Super keys.
- Alternate Key: Alternate Keys or Secondary Keys are not selected as primary keys but as candidate keys. So a key that is candidate key but can work as a primary key is known as an alternate key.
- Composite/Compound Key: A key that can (act as a) be primary or candidate key in a combination of two or more columns/attributes in a table. In simple words, any primary key containing two or more attributes can be termed a composite key. It identifies a row in a table. Candidate key can have one attribute, but composite should have at least two attributes.
- Unique Key: This key is similar to the primary key but can accept one null value without a duplicate value. So the work is the same, but the limitations are slightly less.
- Foreign Key: Key used to link two tables where the table with a primary key is a Parent Table, and the table with a foreign key is the child table. It can accept null and duplicate values.
3) SQL JOINS:
As the name suggests, SQL Joins combine two or more tables based on a common field/attribute/property. It retrieves data from multiple database tables. There are four types of SQL Joins – INNER, FULL, LEFT, and RIGHT Join.
4) Views in SQL:
A SQL query or virtual table that doesn’t store any data displays data stored in a table. A view can be created, updated, or dropped in the following manner:
- CREATE: CREATE VIEW view_name AS;
- UPDATE: CREATE AND REPLACE VIEW view_name AS;
- DROP: DROP VIEW view_name;
Use of VIEW:
Restricting data accessSimplify commandHiding data complexityRename columnsMultiple view facility
5) Subqueries in SQL:
You can also name a Subquery as a nested or inner query. Query within the query is known as a nested query. A subquery can be embedded in three Clause; WHERE, FROM, and HAVING Clause.
They also come with statements like SELECT, UPDATE, INSERT, DELETE, comparison operators (=, <, >, <=, >=), IN and BETWEEN operator. Another critical aspect of the subquery is no usage of the “ORDER BY” command. A subquery is always enclosed in parenthesis. Subqueries tackle unique problems requiring numerous queries to get a specific output.
6) Normalization of Database:
A database contains an enormous amount of data. And, an unorganized database makes retrieval of data troublesome. Thus, keeping data in an organized manner makes the recovery of data easy.
Normalization is a database design technique/approach to organize a database by reducing redundancy and storing it logically. It divides a larger table into small fragments and relates/links them. Various forms of Normalization are 1NF, 2NF, 3NF, and BCNF (Boyce Codd Normal Form).
7) Basic SQL Commands:
SQL or Sequential Query Language is a language to manage data in RDBMS. And SQL Commands are commands for communicating with the database. Every command has a specific significance. There are five basic SQL commands: DML, DDL, DCL, DQL, and TCL. Let’s discuss each.
- DDL: Data Definition Language command is used to CREATE, DROP (delete), ALTER (modify), RENAME, and TRUNCATE (delete all rows) structure of an object. (Creation, modification, deletion of single field, deletion of entire row)
- DML: Data Manipulation Language (DML) is used to modify the database by inserting, updating, or deleting. IT consists of INSERT, UPDATE, and DELETE commands.
- DCL: Data Control Language protects the information from unauthorized access. It can enable or disable access to the power of a user. It consists of commands such as GRANT and REVOKE.
- TCL: Transaction Control Language works hand-in-hand with the DML command. Commands to manage transactions in the database such as COMMIT, ROLLBACK, and SAVEPOINT.
- DCL: Data Query Language is used to retrieve data from the database. It consists of only one command, i.e., SELECT.
8) Transactions in SQL:
Set of tasks grouped in a single execution unit. These units are performed against a database. A Transaction begins with a task and only ends if it finishes all the tasks. If a task fails, the transaction fails. Therefore, a transaction can be successful or fail. The main reason for the failure of the transaction is incomplete steps or insufficient data.
To understand this, let’s take an example of an ATM transaction. Here, ATM transaction begins, the ATM checks the balance in the account, if it’s sufficient, writes the withdrawal operation into the log table. If the transaction succeeds, amount modification is written in the database.
If the transaction fails, it rollbacks the data modification to ensure data integrity. The transaction has four properties known as the ACID (Atomic, Consistent, Isolated, and Durable).
9) Aggregate Functions in SQL:
It’s a primary function in SQL but an important concept. An aggregate function groups the multiple rows on specific criteria. Functions like COUNT, SUM, AVG, MIN, MAX are aggregate functions.
Count returns total number, Sum returns the addition, AVG returns average; MIN and MAX return the minimum and maximum value, respectively. Only COUNT can accept a null value. Aggregate functions are typically used with the GROUP BY Clause.
10) NoSQL:
NoSQL refers to Non-SQL or “not only SQL,” which are databases that store data in the form of documents instead of relational tables. NoSQL databases include graph databases, pure document databases, wide column databases, and key-value databases.
NoSQL doesn’t require any specialized Database hardware; it uses commodity hardware.
Few more SQL concepts:
- Types of Table Relationships: One to One, One to Many, and Many to Many.
- Table types: Fact and Dimension tables
- Date-Time Manipulation
- UNION
- String Formatting
- Cloning Tables in SQL
Conclusion
SQL is an essential programming language to interface with the database system. It has been the standard language for RDBMS. It is easy to learn, use and manages massive volumes of data. If you are looking to become a data scientist, analyst, or mining specialist, SQL is a necessary skill to have. Here we have discussed ten foundational concepts of SQL.
