Structured Query Language (SQL) is a standard computer language for relational databases like MySQL, Oracle, and MS Access. The primary purpose of SQL is to store, manipulate and retrieve data from the databases. SQL is used to create, delete and modify databases. SQL also makes it easier for the users to define the data in relational databases, describe it and alter it. In simple words, the main purpose of using the SQL programming language is to communicate with a database.
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Here is what you can use SQL for:
- Executing queries against a database
- Creating new databases
- Creating tables in the database
- Creating views in a database
- Deleting records in a database
- Inserting records into a database
- Retrieving data from a database
- Updating records in a database
- Creating stored procedures and views in the database
- Creating permissions for viewing tables in a database.
The standard SQL is that of the American National Standards Institute (ANSI). However, different versions of SQL comprise major commands like select, delete, update, and insert. The foundation of using SQL is the relational database management system (RDBMS) that stores database objects in the tabular format which is nothing but a collection of different rows and columns that contains information about related data entries.
SQL Tutorial
Before we deep dive into the components of SQL, let us understand the SQL query processing briefly:
An SQL structure contains four main components – a query dispatcher, optimization engine, classic query engine, and SQL query engine.
In query processing, the high-level queries are translated into low-level expressions. All the activities involved in the extraction of data from a database are added in query processing.
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Here is how query processing in SQL works.
-
Parsing and Optimising:-
The first step in query processing is parsing wherein the query is converted into regional algebra, followed by database checks like syntax, semantic and shared pool check (to confirm written hash codes in the pool). The syntax check is used to determine the syntactic validity of the query whereas the purpose of a semantic check is to confirm that the statement has a definite meaning.
Next comes optimisation of the parse, during which the examination of several query examination plans is done to determine the most efficient query plan for the analysis.
-
Execution:-
After the optimizer passes the lowest cost query plan for execution, the execution engine runs the query and displays the final result at the end.
SQL Concepts
The following are some of the most critical SQL RDBMS concepts that you must learn in an SQL Tutorial.
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Field:-
The entries in a table are subdivided into different categories that contain specific information.
-
Row and column:-
Every individual horizontal entry in a table is called a row Ora record of data whereas vertical entries are referred to as columns.
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Constraints:-
In SQL, constraints mean the rules or limitations applied to data entries in rows or columns. The purpose of using constraints is to restrict the type of data that can be added as entries in a table.
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Primary and foreign keys:-
Primary keys in SQL are unique languages used to uniquely identify rows or columns. A foreign key or reference key is used to link two tables.
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How to Create a New Table in SQL?
Here are the steps you need to follow to create a new table in SQL.
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Specifying new relation:-
The first step to creating a table in SQL is to specify a new relation. You must start by providing a to the relation, mentioning attributes and the initial constraints on data entries. Then you can create base tables.
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Creating schema:-
The next step is to create a schema, which is a list of logical structures in SQL containing database objects like tables, stored procedures, functions, views and triggers.
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Adding information and constraints to the columns:-
The last step is to add information to columns by adding column name, its type, keys, and constraints.
The syntax for creating a table is SQL RDBMS is
CREATE TABLE table_name (
column1 datatype,
column2 datatype,
column3 datatype,
….
);
SQL Syntax
The unique set of rules and guidelines to write statements in SQL is called syntax. The following is the syntax for various statements in SQL.
-
SQL SELECT Statement:
SELECT column1, column2….columnN
FROM table_name;
-
SQL DISTINCT Clause:
SELECT DISTINCT column1, column2….columnN
FROM table_name;
-
SQL WHERE Clause:
SELECT column1, column2….columnN
FROM table_name
WHERE CONDITION;
-
SQL AND/OR Clause:
SELECT column1, column2….columnN
FROM table_name
WHERE CONDITION-1 {AND|OR} CONDITION-2;
-
SQL DELETE Statement:
DELETE FROM table_name
WHERE {CONDITION};
-
SQL ALTER TABLE Statement:
ALTER TABLE table_name {ADD|DROP|MODIFY} column_name {data_ype};
-
SQL INSERT INTO Statement:
INSERT INTO table_name( column1, column2….columnN)
VALUES ( value1, value2….valueN);
-
SQL CREATE DATABASE Statement
:
CREATE DATABASE database_name;,
- The syntax to create a database in SQL is CREATE DATABASE DatabaseName;
- To drop or delete a database, the syntax is DROP DATABASE DatabaseName;
- The syntax to select a database is USE DatabaseName;
- The syntax to drop a table is DROP TABLE table_name;
- For inserting query in a database, the two syntax that can be used are
- INSERT INTO TABLE_NAME (column1, column2, column3,…columnN)]
- VALUES (value1, value2, value3,…valueN);
- The syntax to select query is SELECT column1, column2, columnN FROM table_name;
- Syntax for AND or OR operators is SELECT column1, column2, columnN
FROM table_name
WHERE [condition1] AND [condition2]…AND [conditionN];
- Syntax to update query is UPDATE table_name
SET column1 = value1, column2 = value2…., columnN = valueN
WHERE [condition];
- Syntax to delete a query is DELETE FROM table_name
WHERE [condition];
- Syntax for sorting results in SQL is SELECT column-list
FROM table_name
[WHERE condition]
[ORDER BY column1, column2, .. columnN] [ASC | DESC];
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SQL Operators
Specific operations in SQL like comparison or arithmetic operations are done with the help of the SQL operator – a reserved character or word in the WHERE clause. They are generally used as conjunctions for adding multiple conditions in a statement.
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Arithmetic operators
- + operator is used to add values on either side of the operator.
- – operator subtracts the right-side value from the left one.
- x operator is used to multiply the values.
- / operator is used to divide the right-hand value from the left one.
- The % operator is used to divide the value and provides a remainder.
-
Comparison operators
- = operator checks whether the two values are equal or not. If the values are the same, the condition becomes true. Example: (a = b) is not true.
- != this operator checks if the values are equal. If the values are not the same, the condition becomes true. Example: (a != b) is true.
- <> this operator also checks if the values are equal. If not, the condition becomes true. Example: (a <> b) is true.
- > this operator is used to check if the left value is greater than the right value.
- < checks if the left value is less than the right value, then the condition is true.
- !< checks if the left value is not less than the right value, then the condition becomes true. Example: (a !< b) is false.
- !> is used to check if the left value is not greater than the right value, if yes, then the condition becomes true.
-
Logical operators
- ALL compares a value to the other values in a set.
- AND is used to create multiple conditions in the WHERE clause.
- ANY compares a value to other values in the list.
- EXISTS is used to search a row in a table under specified conditions.
- UNIQUE is used to search every row of a table to ensure that no value is repeated.
Conclusion
SQL is often used as a data definition and a data manipulation language that allows users to create new databases and make changes in existing relational databases. It is also used to control data to protect it from misuse. Businesses commonly use the SQL language for data analysis, back-end development, and database administration. Therefore, if you are interested in data and want to pursue a career in data science, it is pertinent to know SQL basics.
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