Interrogation and Responses for SQL Queries

2 years ago

14 minutes

SQL interview questions are commonly asked in almost every interview due to the prevalence of database operations in applications. SQL, or Structured Query Language, is a specific programming language used for managing relational databases and communicating with them. It includes a set of standard commands like SELECT, INSERT, CREATE, DELETE, UPDATE, and DROP, which enable users to retrieve and upload data, create and remove table elements, and establish dynamic database interactions between servers and programs.

Can you provide one option to paraphrase the phrase “SQL Interview Questions” natively?

What is the meaning of SQL?

SQL is a specialized coding language used for querying and manipulating data in organized databases. By utilizing SQL commands, you can effectively perform actions such as reading, writing, selecting, and deleting entries or columns within databases. Additionally, SQL offers an efficient method for establishing a dynamic connection between your programs, websites, or mobile apps and a database. For instance, when you input your login credentials on a user website, SQL transfers this login information to a database for authentication and user restrictions.

SQL uses both Database and Relational Database Management Systems (DBMS and RDMS) to store data and structures. However, each type of DBMS is preferred for different purposes. The main difference between them is that DBMS saves information as files, while RDMS organizes it in tabular form. RDMS, as the name suggests, allows relationships between tables using Primary Keys, Foreign Keys, etc. This creates a dynamic chain of hierarchy and offers useful restrictions on the tables. On the other hand, DBMS sorts its tables in a hierarchical or navigational manner. This is beneficial when storing data in independent tables without affecting other tables during the filling or editing process.

SQL is built on top of relational operations, incorporating various modifications and advancements. The most fundamental form of an SQL query is as follows:

select A1, A2, ..., An

from R1, R2, ..., Rm

where P

What are the various types of SQL commands?

SQL commands can be categorized into four groups:

DML (Data Manipulation Language) which provides data manipulation features
DDL (Data Definition Language) which is used to manipulate database structures
TCL (Transaction Control Language) that takes in charge data transaction verification and error handling
DCL (Data Control Language) are security statements that feature user restrictions and data access permissions to promote security of your data.

What purposes does SQL serve?

SQL is commonly employed and favored by server-side programmers due to its capability to swiftly and effortlessly handle a substantial volume of data entries in a database. This brings about significant enhancements in both data extraction and modification. To elaborate further, SQL offers the functionality to execute, retrieve, add, modify, and remove entries from a database. It also enables the creation of structures like tables, views, and databases, given that a distinct name is provided. Additionally, the SELECT, INSERT, CREATE, DELETE, UPDATE, and DROP keywords are used to define specific actions in SQL.

SELECT keyword is used to highlight and get entries in rows from tables or views. It can also be accompanied by AS keyword to provide an alias. To filter the SELECT statement, WHERE clause may be included to provide filter conditions and select only the wished entries that satisfy the condition.
INSERT allows to add or insert a row or multiple rows in a database table. Accompanied by VALUES keyword lets you add a row with specific values. INSERT may also be accompanied with SELECT to insert the preselected row.
CREATE is a keyword used to create elements in SQL. It is usually accompanied with the keyword to be created such as CREATE DATABASE, CREATE TABLE, CREATE VIEW, etc.
DELETE keyword is used to deletes record(s) in a database. You should always use it carefully to avoid unwanted data loss. You may delete records you didn’t want to delete. Use WHERE clause to specify the range of the records you wish to delete.
UPDATE keyword updates or changes the existing data within an existing record. Be sure to note that the record must be existent.
DROP keyword drops or deletes a table within the database.

What are the main distinctions between SQL and P/L SQL?

SQL, also known as Structured Query Language, serves as a means of communication with relational databases and allows for the manipulation and creation of databases. On the other hand, PL/SQL, a variation of SQL developed by Oracle Corporation in the early 1990s, enhances the functionalities of SQL by incorporating procedural aspects of programming languages.

DDL or Data Definition Language refers to SQL commands that directly impact the structure of a database. DDL falls under the SQL command classifications which also include DML (Data Manipulation Language), Transactions, and Security. DDL commands have the ability to manipulate various elements like indexes, objects, tables, views, triggers, and more. In SQL, three commonly used DDL keywords are:

Data Manipulation Language (DML) is a set of commands used to manipulate data within a table.

DML, also known as Data Manipulation Language, encompasses a collection of commands that are categorized based on their ability to authorize users to modify database entries. These operations involve inserting, retrieving, deleting, or updating data within tables. The following are examples of commonly used DML statements derived from these essential functionalities:

SELECT – used to highlight a row within a table and retrieve it.
SELECT [columnName] FROM [tableName]
UPDATE – used to update entries from existing tables.
UPDATE [tableName] SET [value]
INSERT – used to insert entries into an existing table.
INSERT INTO [tableName]
DELETE – used to delete entries from an existing table
DELETE FROM [tableName]

What does TCL stand for?

TCL refers to a set of SQL commands that primarily focuses on managing database transactions and save points. These commands allow developers to incorporate their specified SQL functions and logic into the structure and behavior of the database. Some examples of TCL commands include COMMIT, which is used to confirm a transaction, ROLLBACK, which reverts a transaction in the event of errors, SAVEPOINT, which marks a specific point for potential rollback, and SET TRANSACTION, which determines the details of a transaction.

Data Control Language, commonly known as DCL, is responsible for managing the authorization and restrictions provided to users, including the permissions and privileges required for SQL statements. For instance, there are specific DCL keywords such as GRANT, which allows users to access particular databases, and REVOKE, which negates the privileges granted to the user.20. ### Specify the structure of tables and their respective fields within a database.

What are the various types of SQL keys?

In RDMS, keys are an essential component. They are fields that establish a connection between tables and facilitate efficient data retrieval and logging by managing column indexes. There are several types of keys, including:
1. Primary Key – this key is unique and serves to identify records in database tables. Unique means that it cannot be Null and must only appear once in the table.
2. Candidate Key – this unique field independently identifies a column or group of columns without the need for reference to other fields.
3. Alternate Key – while it can be used as a substitute for Primary Keys, it is considered secondary. The difference is that Alternate Keys can have a Null value, as long as the columns contain data within them. They are also required to be unique and function as a type of Candidate Key.
4. Unique Key – these keys prevent duplicate data within rows, excluding null entries.
In addition to these, there are other types of keys in RDMS such as Foreign Keys, Super Keys, and Composite Keys.

-To name the various types of indexes in SQL and explain them.

Unique Index ensures that there are no duplicate entries in columns with unique indexing and is automatically created if a Primary Key is present. Clustered Index is utilized to structure or modify the arrangement of data in the table based on the key value. Each table can have only one clustered index. On the other hand, NonClustered Index solely controls the logical order of data entries, without impacting the table arrangement, and multiple NonClustered Indexes can be present in a table.

The SQL language, also known as Standard Query Language, is a programming language used for interacting with fields and columns in a database. MySQL, on the other hand, is a type of Database Management System, specifically an RDMS or Relational Database Management System. Although not a programming language itself, MySQL does implement the SQL syntax. Now, let’s move on to the explanation of the UNION and UNION ALL keywords in SQL and their distinctions.

The SQL UNION operator merges multiple sets specified in the SELECT statements. The criteria for the sets are: (1) identical column count, (2) matching data types, and (3) the same column ordering in the SELECT statements. Duplicate rows are automatically removed from the result set. UNION ALL performs the same task as UNION, but includes all rows, including duplicates.

SELECT C1, C2 FROM T1
UNION
SELECT Cx, Cy FROM T2;

What are the various kinds of joins available in SQL?

The join keyword retrieves records from multiple tables by utilizing various keys to locate these records, ensuring a connection between fields.

Right Join: Retrieves rows from the right-hand side table, also including the shared rows.

Left Join: Retrieves rows from the left-hand side table, also including the shared rows.

Full Join: Retrieves all rows, irrespective of their presence in both tables.

What is the concept of Normalization and Denormalization?

Normalization organizes the tables and their fields in a database to minimize duplication, simplifying the tables while preserving unique fields. On the other hand, denormalization retrieves fields from all forms of normalization, introducing redundancies into the table.

Both clauses allow conditions to be used as the criteria for retrieving fields. The distinction, however, lies in the fact that the WHERE clause is specifically used for static non-aggregated columns, whereas the HAVING clause is solely for aggregated columns.

select order_id, SUM(sale_amount) as TotalSale 
from SalesData 
where quantity>1 
group by order_id 
having SUM(sale_amount) > 100;

What separates UNION, MINUS, and INTERSECT?

In SQL, the UNION keyword is utilized to combine multiple SELECT queries while excluding duplicates in the result set. The INTERSECT keyword is solely employed to retrieve rows that are common between multiple tables using SELECT queries. On the other hand, the MINUS keyword subtracts the results of two SELECT queries, providing the difference between the first and second queries. This means that any row present in both result sets will be eliminated from the final output. As for selecting 10 records from a table, refer to the steps below:

To select 10 records from a table, you can use the following SQL query:

SELECT *
FROM table_name
LIMIT 10;

MySQL: To ensure the integrity of your database when deleting an element from one table causing the deletion of related elements in another table, you can configure foreign key constraints with the “ON DELETE CASCADE” option.

Oracle: To maintain database integrity in situations where deleting an element in one table might delete related elements in another table, you can set up referential integrity constraints using the “ON DELETE CASCADE” clause.

SQL Server: If you need to maintain the integrity of your database when deleting an element from one table which may result in the deletion of related elements from another table, you can utilize foreign key relationships with the “ON DELETE CASCADE” option.

How can data from Table A be copied to Table B?

INSERT INTO TableB (columnOne, columnTwo, columnThree, ...)

SELECT columnOne, columnTwo, columnThree, ...

FROM TableA

WHERE added_condtion;

What distinctions exist between the IN clause and the EXISTS clause?

The main distinction between the two is that the EXISTS keyword executes faster compared to the IN keyword. This is because the IN keyword needs to search through all the existing records, whereas the EXISTS keyword automatically stops once a matching record is found. Moreover, the IN Statement works within the ResultSet, while the EXISTS keyword operates on virtual tables. Additionally, the IN Statement does not function with queries associated with Virtual tables, while the EXISTS keyword is used with linked queries.43. ### What is the meaning of the acronym ACID in Database Management?

The ACID Acronym represents the principles of Atomicity, Consistency, Isolation, and Durability. This characteristic is responsible for ensuring the integrity of the database system, ensuring that any data transaction issued by the user is executed entirely, accurately, and remains persistent.

A database trigger is a native program that is automatically executed when a specific event occurs on a table or view, such as the insertion, update, or deletion of a record. The primary purpose of a database trigger is to ensure the consistency and integrity of the database.

The feature of auto increment allows users to generate a distinct number every time they insert a new record into a table. In Oracle, the keyword for auto increment is AUTO INCREMENT, while in MySQL it is AUTO_INCREMENT, and in SQL SERVER the IDENTITY keyword can be used for auto-incrementing. This keyword is primarily used to establish the primary key for the table.

To put it simply, collation refers to a set of guidelines that determine how characters are compared and organized within strings. In MSSQL and MySQL, collation functions similarly, except for specific options like UTF-8. Apart from the typical character-based comparison, collation can also assess and arrange strings based on their ASCII representation.

One possible paraphrase of the sentence could be:
“A stored procedure that invokes itself repeatedly until it meets a specific condition. This recursive function or procedure allows developers to reuse the same block of code as many times as needed.”

The solution lies in using the LIKE operator, which serves the purpose of pattern matching and can be utilized as -.

% – Matches zero or more characters.
_(Underscore) – Matching exactly one character.

Can you explain what Hibernate is and how it relates to SQL?

Hibernate is a Java-based tool used for Object Relational Mapping. It allows developers to write code in an object-oriented manner, and then converts it into SQL queries that can be executed on a relational database. Hibernate has its own language called Hibernate Query Language (HQL) which enables querying of Hibernate’s entity objects. Additionally, Hibernate offers a Criteria Query, which is an object-oriented query language within Hibernate. This feature is particularly beneficial for developers who predominantly work with objects in their front-end applications. With Criteria Query, developers can incorporate SQL-like functionalities such as security and restriction access.

Typically, this mistake arises from incorrect syntax when calling a column name in an Oracle database. Pay attention to the ORA identifier displayed in the error code. Ensure that you have entered the accurate column name. Additionally, be mindful of the aliases, as they are the ones identified as the invalid identifier causing the error.53. ### What role does a SQL Profiler serve?

The SQL Profiler is a user-friendly interface that enables database developers to observe and monitor the activities of their database engine. It records all events and offers analysis to identify issues and inconsistencies. Essentially, it serves as a diagnostic tool in SQL that effectively troubleshoots performance problems and allows for a comprehensive view of the specific part of a trace file that is causing disruptions in SQL transactions.

To establish a connection between these two, you will need a JDBC (Java Database Connectivity) driver. Additionally, in your build.gradle file, you need to include the appropriate dependencies, permissions, and grants.

Here is one possible paraphrase:

Source: Wikipedia