10 Essential SQL Queries Every Developer Should Know

Introduction

SQL (Structured Query Language) is a programming language used to interact with relational databases. It can manage and modify database structures, as well as store, retrieve, and manipulate data. With the help of SQL commands, users can easily generate formatted reports for presentations in professional settings. SQL is at the core of many database-related technologies and programs.

What is SQL

SQL was developed by IBM in 1977 and has become an important tool in the IT field, especially in enterprises that need to work with database data. Since the 1980s, SQL has continued to grow in popularity and is widely used in relational database management systems (RDBMS). Initially, SQL was designed as a data manipulation language for IBM mainframes, but now it has been integrated into multiple platforms and programming languages, such as Java, C#, and .Net.

10 SQL concepts every developer should know

1. SQL is the core of relational databases: There are different types of database systems, such as relational, hierarchical, and document databases. SQL is dedicated to operating relational databases. RDBMS is the abbreviation of Relational Database Management System, which is the basis of SQL and all modern database systems such as MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access. In short, RDBMS stores data in the form of tables, where tables are composed of rows and columns, and each record in the table corresponds to a data entry in the database.
2. Keys in SQL: A key is a single field or a combination of multiple fields in a table, which is used to extract a specific record or row from a data table. Keys are also responsible for linking two or more tables or views. Common SQL keys include the following:

• Primary key: A primary key is one or more fields in a table that can provide a unique identifier for each record in the table. The values in the primary key must be unique, and a table can only have one primary key. A composite primary key is a primary key composed of multiple fields.
• Foreign Key: A foreign key is a key used to connect two tables. It establishes a relationship between tables by matching the primary key in one table with the foreign key in another table.
• Unique Key: A unique key is a field or group of fields used to ensure that each record has a unique identifier. Although similar to a primary key, a unique key allows one null value and does not allow duplicate values.

3. Views in SQL:

• The role of views: A view is a virtual table that displays data from one or more tables without directly storing any data. Therefore, a view is actually equivalent to a result set of a SQL query. A view can contain part or all of the data in a table and provide filtering and protection for specific data. By using views, you can control user access rights to ensure that they can only see the required data. Views help hide sensitive information and restrict users to access only specific data to enhance data security.

4. SQL Join: A join is an operation that combines data from two or more tables into a single result set. Common join types include:

• INNER JOIN: Returns only matching records that exist in both tables.
• LEFT JOIN (or LEFT OUTER JOIN): Returns all records from the left table, even if there are no matching records in the right table.
• RIGHT JOIN (or RIGHT OUTER JOIN): Returns all records from the right table, even if there are no matching records in the left table.
• FULL OUTER JOIN: Returns all records from both tables, regardless of whether there are matches.

5. Database normalization: Normalization is a method of organizing tables and columns in a database to eliminate duplicate data and ensure that each piece of data is used only once. Normalization is usually divided into three levels:

• First Normal Form (1NF): Ensure that the data in each column is atomic, that is, it cannot be divided.
• Second Normal Form (2NF): On the basis of meeting 1NF, ensure that all non-primary key fields depend on the primary key.
• Third Normal Form (3NF): On the basis of meeting 2NF, eliminate the transitive dependency of all fields on non-primary key fields.

6. Transactions in SQL: A transaction is a set of SQL statements that must all succeed or all fail. Transactions ensure the integrity and consistency of the database even when the application stops unexpectedly. SQL provides multiple commands to manage the life cycle of transactions, including starting (BEGIN), committing (COMMIT), rolling back (ROLLBACK), and setting transaction boundaries.

7. Subqueries in SQL: Subqueries are queries nested within a single query to retrieve data from a table. They can be used in conjunction with views or joins. When the results of one SELECT statement are used in another SELECT statement, this nested SELECT is called a subquery. For example, if the primary key of a table is customer ID and the foreign key is order ID, a subquery can be used to retrieve related orders based on customer ID.

8. Clone a table in SQL: Sometimes you need to create an exact copy of an existing table for experimentation or other operations without affecting the original table. In MySQL, this can be achieved by following the steps below:

• First, use the SHOW CREATE TABLE command to get the CREATE TABLE statement of the source table, which contains the table's structure, indexes, and other important details.
• Then, modify the statement to replace the table name with the name of the new cloned table. After executing the modified statement, you will get an identical cloned table.
• If you want to copy the contents of the original table to the new table as well, you can add an INSERT INTO or SELECT statement after the clone.

Suppose we have a source table students, and we want to clone this table and name it students_clone.

Step 1: Use SHOW CREATE TABLE to get the creation statement of the source table

SHOW CREATE TABLE students;

CREATE TABLE `students` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(100) DEFAULT NULL,
  `age` int DEFAULT NULL,
  `grade` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`)
);

Step 2: Change the table name and create a clone table

Now, we can change students to students_clone and then execute this statement to create a clone table:

CREATE TABLE `students_clone` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(100) DEFAULT NULL,
  `age` int DEFAULT NULL,
  `grade` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`)
);

Step 3: Insert the data of the original table into the cloned table

If you want to copy the data of the original table to the new table, you can use the INSERT INTO or SELECT statement to insert the content into the new table:

INSERT INTO students_clone (id, name, age, grade)
SELECT id, name, age, grade FROM students;

9. SQL sequence: A sequence is a collection of numbers that are generated in an incremental manner. It is often used to ensure the uniqueness of fields in a database table. When using an auto-increment column, you can get the values in the sequence row by row and compare them with other records to check for duplication.

10. Temporary tables in SQL: Relational database management systems (RDBMS) support temporary tables. When using temporary tables, you can take advantage of the same select, update, and join capabilities as standard SQL Server tables, which is a major advantage. An important point is that temporary tables are automatically deleted at the end of the client session, ensuring that no additional storage space is occupied.

Conclusion

Mastering these basic concepts will enable you to write effective SQL queries and effectively manage data in your database. This is a critical skill for data analysts, database administrators, and software developers working with relational databases. As your skills improve, you may be exposed to more advanced SQL concepts such as subqueries, window functions, and common table expressions.

By mastering these ten basic concepts, you will be well on your way to becoming a proficient SQL user. It is important to note that SQL is an evolving language, and staying up to date with the latest developments and best practices is essential to ensuring that your SQL code is efficient and effective.

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