Are you ready to take your SQL Developer interview and looking for SQL Interview Questions? You’re at the right spot. This guide will allow you to brush up on your SQL abilities, boost confidence, and prepare for your next job!
You will find an array of real-world Interview questions from organizations such as Google, Oracle, Amazon, Microsoft, and many more. Each question has an answer that is written in line, which will save you interview time.
Additionally, it covers exercises to help you comprehend the basics of SQL.
A database is an organized data set kept and accessible digitally from a local or remote computer system. They can be large and complicated, and these databases are created with a fixed design and modeling methods.
DBMS refers to Database Management System. DBMS is a software system responsible for creating, retrieving, updating, and administering databases. It makes sure that the information is organized, consistent and accessible through its connection between the Database’s Database as well as its users or applications software.
RDBMS refers to Relational Database Management System. The main difference in this in comparison to DBMS is that RDBMS records information in the form of tables, and also, relations can be created between the fields common to these tables. Most modern database management systems, such as MySQL, Microsoft SQL Server, Oracle, IBM DB2, and Amazon Redshift, are built on RDBMS.
SQL is a short form in form of Structured Query Language. It is the language of choice for relational database management systems. It is particularly helpful in managing organized data composed of entities (variables) and relationships between the various data types.
SQL is a common language to retrieve and manipulate structured databases. In contrast, MySQL is a relational database management system similar to SQL Server Oracle and IBM DB2 and IBM DB2 that helps manage SQL databases.
A table is a logical storage of data in columns and rows. Columns can be classified as vertical, while rows are horizontal. The columns of tables are known as fields, while rows could be called records.
Constraints are used to define the rules for data that are contained within the table. They can be used for one or more fields within an SQL table when it is created or after it has been created with the command ALTER TABLE. The restrictions are:
The primary KEY constraint is unique and uniquely identifies every table row. It should contain unique values, and an implied NOT NULL control. A table in SQL is strictly limited to only having one primary key. This comprises one field or several fields (columns).
CREATE TABLE Students ( /* Create table with a single field as primary key */ID INT NOT NULL Name VARCHAR(255) PRIMARY KEY (ID) ); CREATE TABLE Students ( /* Create table with multiple fields as primary key */ID INT NOT NULL LastName VARCHAR(255) FirstName VARCHAR(255) NOT NULL, CONSTRAINT PK_Student PRIMARY KEY (ID, FirstName) ); ALTER TABLE Students /* Set a column as primary key */ADD PRIMARY KEY (ID); ALTER TABLE Students /* Set multiple columns as primary key */ADD CONSTRAINT PK_Student /*Naming a Primary Key*/PRIMARY KEY (ID, FirstName);
A UNIQUE constraint makes sure that the values of the column are unique. This ensures the uniqueness of the column(s) and aids in identifying each row individually. Contrary to primary keys, they can have multiple unique constraints that can be defined for each table. The syntax used for Unique is very identical to the syntax used for PRIMARY KEY and is employed interchangeably.
CREATE TABLE Students ( /* Create table with a single field as unique */ID INT NOT NULL UNIQUE Name VARCHAR(255) ); CREATE TABLE Students ( /* Create table with multiple fields as unique */ID INT NOT NULL LastName VARCHAR(255) FirstName VARCHAR(255) NOT NULL CONSTRAINT PK_Student UNIQUE (ID, FirstName) ); ALTER TABLE Students /* Set a column as unique */ADD UNIQUE (ID); ALTER TABLE Students /* Set multiple columns as unique */ADD CONSTRAINT PK_Student /* Naming a unique constraint */UNIQUE (ID, FirstName);
A FOREIGN KEY is one or a collection of fields within an individual table that refers to the primary KEY in a different table. Foreign vital constraints ensure the integrity of the relationship between tables.
The table subject to the constraint on foreign keys is marked the child table, and the table that holds the critical candidate is identified as the parent or referenced table.
CREATE TABLE Students ( /* Create table with foreign key - Way 1 */ID INT NOT NULL Name VARCHAR(255) LibraryID INT PRIMARY KEY (ID) FOREIGN KEY (Library_ID) REFERENCES Library(LibraryID) ); CREATE TABLE Students ( /* Create table with foreign key - Way 2 */ID INT NOT NULL PRIMARY KEY Name VARCHAR(255) LibraryID INT FOREIGN KEY (Library_ID) REFERENCES Library(LibraryID) ); ALTER TABLE Students /* Add a new foreign key */ADD FOREIGN KEY (LibraryID) REFERENCES Library (LibraryID);
Self-JOIN a self-join is a type of regular join in which the table is joined to itself by an association between the table’s column(s). Self-join is a type of join that uses the INNER LEFT JOIN or JOIN clause and the table alias to assign various names to the table in the query.
SELECT A.emp_id AS "Emp_ID",A.emp_name AS "Employee", B.emp_id AS "Sup_ID",B.emp_name AS "Supervisor" FROM employee A, employee B WHERE A.emp_sup = B.emp_id;
Cross-join is defined as a cartesian product from the two tables in the join. The table that follows join has the same amount of rows as the cross-product of the number of rows between the two tables. If a WHERE condition is employed in cross-join, the query works as an INNER join.
SELECT stu.name, sub.subject FROM students AS stu CROSS JOIN subjects AS sub;
An index in a database is a data structure that provides a quick search of data within one or more columns in tables. It improves performance when accessing the data in a database table but at the expense of other writing and memory to keep the index’s data structure.
CREATE INDEX index_name /* Create Index */ON table_name (column_1, column_2); DROP INDEX index_name; /* Drop Index */
As previously explained, The differences can be broken down into three minor elements Three factors.
Data Integrity assures the accuracy and reliability of the data throughout its entire life cycle and is an essential element in the design of the system, its implementation, and the use of any system that manages stores, and retrieves data. It also establishes integrity constraints to ensure that business rules adhere to the data once it has been added to an application or Database.
An inquiry is a request to retrieve information or data from a table in a database or a combination of tables. A query for a database could be either a query that is a select or an action query.
SELECT fname, lname /* select query */FROM myDb.students WHERE student_id = 1;
UPDATE myDB.students /* action query */SET fname = 'Captain', lname = 'America' WHERE student_id = 1;
Subqueries are within another query, referred to as interspersed or inner queries. It is utilized to limit or increase the amount of data the primary question can access, thereby limiting or improving the results of a direct query. For instance, in this case, we get the contact information for students who have registered in the maths subject.
SELECT name, email, mob, address FROM myDb.contacts WHERE roll_no IN ( SELECT roll_no FROM myDb.students WHERE subject = 'Maths');
The subquery has two kinds of subqueries: correlated and non-correlated.
The UNION operator blends and returns the result set retrieved from two or more select statements.
The MINUS operator within SQL can eliminate duplicates in the result set compiled from the second query. It filters the results obtained through the first SELECT query and then returns results filtered by the first.
The INTERSECT clause of SQL blends the result set obtained by two SELECT statements, where the results from one match those from the other and returns this intersection of the result sets.
Specific requirements must be satisfied before the execution of either or both of these statements using SQL –
The database cursor can be described as a structure that permits the traversal of records within databases. Cursors also aid in processing following traversal, for example, retrieval, adding to the removal of database records. They are viewed as a way to point to a row within a set of rows.
working in conjunction with SQL Cursor
DECLARE @name VARCHAR(50) /* Declare All Required Variables */DECLARE db_cursor CURSOR FOR /* Declare Cursor Name*/SELECT name FROM myDB.students WHERE parent_name IN ('Sara', 'Ansh') OPEN db_cursor /* Open cursor and Fetch data into @name */ FETCH next FROM db_cursor INTO @name CLOSE db_cursor /* Close the cursor and deallocate the resources */DEALLOCATE db_cursor
Normalization is the process of arranging structured data in the Database effectively. It includes the construction of tables, creating relationships to them and creating rules for these relationships. Redundancy and inconsistent data are controlled using these rules, thereby providing flexibility to databases.
Denormalization is the reverse process to normalization. In this process, the normalized schema transforms into a schema with redundant data. Performance is improved by making use of redundancy, and by making sure that the redundant data is consistent. The denormalization is to reduce the amount of overhead created in the query processor due to being over-normalized.
If a table is deleted, the table’s associated items are removed too. This includes the relationships that are defined in the table’s relation to other tables, integrity checks and restrictions, access privileges, and any other rights that the table is granted. To create and utilize the table in its original format, the entire set of relations and constraints checks privileges, and relationships must be rewritten. If the table is cut down, it is not affected by the above issues arise, and the table remains in its original form.
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The TRUNCATE command can be used to remove all rows in the table and clear the space that is occupied by the table.
It is a command that deletion command only deletes rows of the table according to the condition specified in the where clause, or erases all rows of the table even if no conditions are set. However, it doesn’t free the table’s space.
Aggregation performs operations on a set of values, resulting in one scalar number. These functions are typically utilized in conjunction with the GROUP BY clause and the HAVING clause of the statement. These are the most frequently employed SQL aggregate function:
NOTE: All aggregate functions discussed above ignore NULL values, except the COUNT function.
A scalar function produces a unidimensional value that is dependent on the input. These are the most frequently employed SQL functions for scalars:
SQL’s user-defined functions are like those in other programming languages that accept parameters, execute complicated calculations, and return the result. They are designed to utilize logic in a repetitive manner whenever it is needed. There are two kinds of SQL user-defined functions:
OLTP is a shorthand in the sense of Online Transaction Processing, an area of the software that can support transaction-oriented software. One of the essential characteristics of an OLTP program is the capacity to ensure that concurrency is maintained. To prevent one-off failures, OLTP systems are often distributed. They are generally built to handle a lot of users performing brief transactions. Database queries are usually uncomplicated, require minimal response time, and produce only a few records. Here’s a look at the functioning of an OLTP system.
Collation is an established set of rules that govern the method by which data is sorted and to be compared. Rules that define the proper sequence of characters are employed to sort feelings. It includes options to specify the sensitivity of the case, accent marks, kana-type characters, and the width of characters. Here are the various types of collation sensitiveness:
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