Most Popular SQL Interview Questions
Modernization of the way we used to maintain records has changed drastically, a lot of data being generated nowadays are in the form of pictures or videos that have become our sole source of keeping memories intact for many years to come. Similarly, multiple organizations and firms have employed a similar method in storing the company data in the form of digitized documents stored away in Database Management Systems (DBMS) which require a special style of language to mine or extract data that the company wishes to extract from its database and here where SQL comes in.
Structured Query Language (SQL) is a domain-specific programming language that is utilized by skilled professionals to manage data stored in the company’s database. SQL skills are in high demand in the market and serve as the foundational basics for any professional looking for a job or brighter prospects in the data industry. Here are a set of SQL Interview Questions that we believe you should prepare for SQL before going for an interview.
Database Management System (DBMS) is software that is solely responsible for creating, controlling, maintenance, and use of a database. DBMS may be defined as a folder that manages data in a database rather than saving a file in the system.
RDBMS abbreviates for Relational Database Management System. It is categorized with the storage of data into a compilation of tables, which is linked by similar topics between the columns of a table. It aids the user with relational operators to influence the data stored in the tables.
SQL abbreviates to Structured Queried Language and aids the operator to communicate with the database. It is a standard operating language that helps execute responsibilities such as recovery, updating, incorporating, and expunging data from the database.
It is a structured table of data made to easily access, store, retrieve, and manage data.
It is a multi-threaded, multiuser structured query language database management system with more than 11 million installations across the globe. The language is the second most well-known and popularly used open-source database programming in use.
MySQL is an oracle sponsored relational database management system (RDBMS) built on structured query language. It is supported by several operating systems which include Windows, LINUX, iOS, etc.
C & C++ are the languages in which MySQL has been written
Below are the technical specifications of MySQL:-
There are 4 noticeable difference between a database & a table:-
A table is a compilation of cells that are structured in a model which eventually form tables and rows. Columns may be categorized as a vertical collection of cells and rows may be categorized as a horizontal collection of cells.
There is also a reference to the cells laid out in a column to create an entity also termed as field once a header is provided to the so-called column.
A field may have several rows which may constitute a record.
Table name:- Employee
Field name’s:- Emp ID, Emp Name, Date of Birth
Data:- 2866, Daniel Decker, 29/02/1984
Below are some of the reasons as to why MySQL server is so famous with its users:-
There are majorly 5 tables present in MySQL: –
There multiple ways of installing MySQL in one’s system, but the best way to do it is manual. The manual installation allows the user to gain a better understanding of the system and aids in additional grasp over the database. There are several benefits linked to the manual installation of MySQL:-
In WINDOWS MySQL command-line tool shows the version information without using any flags, but for a piece of more detailed information the operator may always feed in the below-mentioned command
MySQL> SHOW VARIABLES LIKE “%version%”.
and it will show a detailed discretion of the version of SQL that the user is using.
Several cells in a table are what may constitute a column and a set of cells in a column constitutes a row. To add columns in MySQL, the following statement of ALTER TABLE may be used:
The drop table statement not only removes the data in the table, but it also removes the structure and definition from the database permanently. Thus, the user needs to be extremely careful whilst using this command, the reason is if once deleted there is no recovery option in MySQL. The command is as follows:-
DROP TABLE table_name
Primary Key may be described as a compilation of fields that meticulously define a row. This is a Unique Key and has an unspoken NOT NULL constraint, implicating Primary keys cannot have NULL values.
This key provides a separately pre-defined constraint that exclusively distinguishes every record in the database which insinuates a distinctiveness for the column or the set of columns.
This is a key that can be linked to the Primary Key of another table. Connections need to be fabricated between the two tables by providing a reference to the foreign key with the primary key of another table.
It is a keyword utilized to question data from multiple tables established on the connections between the fields of the table. Keys play a crucial part when JOIN’s are employed.
JOIN’s are tools that help the user retrieve data and depend on the links between tables. Following are the types of ‘JOIN’ used in SQL:-
It is the procedure of reducing redundancies and dependencies by structuring fields and tables of a database. The primary motive of ‘Normalization’ is to add, modify, and delete that can be merged into a single table.
It is a method employed to gain access to data from higher to lower normal types of database. It is also a way of implementing redundancy into a table by integrating data from the correlated tables.
Normalizations may be dissected into 5 forms:-
The view is a computer-generated table that comprises a subsection of data enclosed in a table. Views are NOT virtually present and require a lesser amount of storage capacity. The view can have data of one or more tables pooled in one and depends on the connection.
An index is a routine tweaking method to permit faster reclamation of records from a table. An Index designs an entry for every value which makes data recovery quicker.
There are primarily 3 types of Index’s:-
A database cursor is a command which facilitates a cross-over of the rows/records in a table. This may be visible as a hint to one row in a collection of rows. It is extremely useful for traversing the retrieval, addition, and removal of database archives.
It is defined as the link between the tables in a database. There are several database-based relationships, and they are as follows:-
A database query is a code created to recover information from the database. The query may be fabricated in a way to match the user’s expectation of the result set which may simply be a question to the database.
As the word describes, it is a query inside a query. The exterior query is known as the – Main Query and the innermost query is called a Subquery. Subqueries are forever implemented first and the outcome from the Subquery is then passed on to the main query.
There are majorly 2 styles of subqueries:-
This procedure is a function that comprises several SQL statements to access the DBMS. Multiple SQL statements are compiled into a ‘Stored Procedure’ and maybe employed anywhere as per requirement basis.
The trigger is a code that automatically executes with some event on a table or with a view in a database. E.g., On the joining of a new hire, new records need to be entered in fields like employee ID, Name, Date of birth, etc.
DELETE command is utilized to delete rows from the table and a WHERE clause may be applied for a provisional set of considerations. Commit and Rollback may be executed post deletion of the statement.
TRUNCATE deletes every row from the table. Truncate control cannot be turned backward.
Local variables are the variables that can be applied or occur within the function. They are unknown to the other functions and cannot be referred to or utilized. Variables can be established whenever the functions are called.
Global variables are the variables that can be employed or be present all over the program. Identical variable proclaimed in a global variable cannot be utilized in functions. Global variables cannot be established whenever a particular function is called.
A constraint may be employed to restrict the data type of a table. It may also be specified at the time of creating or altering the table. Some examples of constraints are:-
It defines the precision and consistency of the data stored in a database. It may also identify integrity constraints to implement business guidelines on the data when it is registered into the application or database.
This enables the operator to fabricate a new number to be generated when a record is inserted into the table. AUTOINCREMENT keyword may be used in Oracle and IDENTITY keyword may be used in an SQL SERVER.
This keyword is used when the primary key is used.
Cluster Index is employed for the convenient recovery of data from the database by adjusting the way the records are stored. A database sorts out rows by columns which are destined to be Clustered Index.
Non-Clustered Index does not adjust the way data was stored in the database, rather fabricates a completely different entity inside the table. It usually points back to the original table rows after investigating.
It is a fundamental storehouse of data from numerous data sources. Data are collected, transformed, and made available for mining and online processing. Warehouse data have a subcategory of data dubbed as Data Mart.
It is a query employed to evaluate itself. It is utilized to assess values in a column with other values in the same column and table.
Cross join describes as a query to calculate the results of the number of rows in the first table multiplied by several rows in the second table. If a WHERE clause is applied in a cross join, then the query will act like an INNER JOIN.
Q44. Describe User Defined functions and their types?
User-defined functions are fabricated to create logic whenever required. It is not required to write the same logic multiple times. Rather, the function may be called or deployed at any given point in time.
There are 3 styles of User-defined functions:-
It describes as a compilation of guidelines that establish how character data may be categorized and compared. It can be used to assess A and other language characters, also depending on the width of the characters.
all values may be used to compare these character data.
These are the different types of collation sensitivities:-
A stored procedure that demands by itself until it achieves some kind of boundary condition. This recursive function or procedure helps computer operators use the identical set of codes ‘n’ number of times.
This is the key to linking one or more tables together in MySQL. It helps in matching the primary key field of another table to connect the two tables. It allows the user to a parent-child relationship within the tables. This can be executed either way:-
Following is the syntax used to define a foreign key using CREATE or ALTER TABLE[CONSTRAINT constraint_name]
FOREIGN KEY [foreign_key_name] (col_name, …)
REFERENCES parent_tbl_name (col_name, …)
To do so the initial step would be to:-
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