SQL is the most popular database and many popular websites use SQL. So, here is a collection of the most asked #SQL interview questions to help you prepare.
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A database is an organized collection of data, stored and retrieved digitally from a remote or local computer system. Databases can be vast and complex, and such databases are developed using fixed design and modeling approaches.
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DBMS stands for Database Management System. DBMS is a system software responsible for the creation, retrieval, updating and management of the database.
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SQL stands for Structured Query Language. It is the standard language for relational database management systems. It is especially useful in handling organized data comprised of entities and relations between different entities of the data.
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SQL is a standard language for retrieving and manipulating structured databases. On the contrary, MySQL is a relational database management system, like #SQL Server, Oracle , that is used to manage SQL databases.
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A table is an organized collection of data stored in the form of rows and columns. Columns can be categorized as vertical and rows as horizontal. The columns in a table are called fields while the rows can be referred to as records
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The PRIMARY KEY constraint uniquely identifies each row in a table. It must contain UNIQUE values and has an implicit NOT NULL constraint.
A table in SQL is strictly restricted to have one and only one primary key.
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A UNIQUE constraint ensures that all values in a column are different. This provides uniqueness for the column(s) and helps identify each row uniquely. Unlike the primary key, there can be multiple unique constraints defined per table.
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A FOREIGN KEY comprises of single or collection of fields in a table that essentially refers to the PRIMARY KEY in another table. Foreign key constraint ensures referential integrity in the relation between two tables.
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The SQL Join clause is used to combine records (rows) from two or more tables in a SQL database based on a related column between the two.
Types -
INNER JOIN
LEFT OUTER JOIN
RIGHT OUTER JOIN
FULL OUTER JOIN
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A query is a request for data or information from a database table or combination of tables. A database query can be either a select query or an action query.
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Create database db_name -
=> CREATE DATABASE db_name;
Drop database db_name -
=> DROP DATABASE db_name;
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CREATE TABLE table_name (
column1 datatype,
column2 datatype,
column3 datatype,
....
);
DROP TABLE table_name;
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If a table is dropped, all things associated with the tables are dropped as well. However, if a table is truncated, none of the above problems exist and the table retains its original structure.
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The DISTINCT statement is used with the SELECT statement. If a record contains duplicate values then DISTINCT will select different values among duplicate records.
Syntax-
SELECT DISTINCT column_name
FROM table_name;
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In order to maintain consistency in a database, before and after the transaction, certain properties are followed. These are called ACID properties.
- Atomicity
- Consistency
- Isolation
- Durability
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SQL Aggregate functions calculate values from multiple columns and return a single value. There are 7 aggregate functions in SQL:
1. AVG()
2. COUNT()
3. MAX()
4. MIN()
5. SUM()
6. FIRST()
7. LAST()
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A View can be defined as a virtual table that contains rows and columns with fields from one or more tables.
Syntax:
CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
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SQL Injection is a type of database attack technique where malicious SQL statements are inserted into an entry field of the database in a way that once it is executed, the database is exposed to an attacker for the attack.
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Normalization is used to organize the data tor remove redundancies .
5 type of normal form:
First Normal Form(1NF)
First Normal Form(2NF)
Third Normal Form(3NF)
Fourth Normal Form(BCNF)
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The relationship can be defined as the connection between more than one table in the database.
Type of Relationship -
1. One to One
2. Many to One
3. Many to Many
4. One to Many
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To select all the records from the table we need to use the following syntax:
SELECT * FROM table_name;
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DELETE removes some or all rows from a table based on the condition.
TRUNCATE removes ALL rows from a table by deallocating the memory pages.
DROP command removes a table from the database completely.
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MINUS – returns all distinct rows selected by the first query but not by the second.
UNION – returns all distinct rows selected by either query
UNION ALL – returns all rows selected by either query, including all duplicates.
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