Introduction to SQL

SQL Commands

SQL statements are categorized into five main types: Data Query Language (DQL), Data Manipulation Language (DML), Data Definition Language (DDL), Data Control Language (DCL), and Transaction Control Language (TCL).

Data Query Language (DQL)

DQL statements are used to fetch data from the database.

• SELECT: Retrieves records from one or more tables.

  • Clauses/Operators: WHERE, ORDER BY, GROUP BY, HAVING, DISTINCT, LIMIT/FETCH/SELECT TOP, OFFSET, UNION, UNION ALL, INTERSECT, EXCEPT/MINUS, LIKE, BETWEEN, IN, EXISTS, ANY, ALL, CASE.

Data Manipulation Language (DML)

DML statements are used to insert, update, and delete data within a database.

• INSERT: Adds new rows of data into a table.

• UPDATE: Modifies existing data within a table.

• DELETE: Removes rows from a table.

• MERGE: Performs INSERT, UPDATE, or DELETE operations on a target table based on a source table's data.

Data Definition Language (DDL)

DDL statements define and manage the structure or schema of the database and its objects (tables, indexes, views, etc.).

• CREATE: Creates a new database object (e.g., CREATE TABLE, CREATE DATABASE, CREATE INDEX, CREATE VIEW, CREATE PROCEDURE).

• ALTER: Modifies an existing database object (e.g., ALTER TABLE to add or drop columns).

• DROP: Deletes an entire database object (e.g., DROP TABLE, DROP DATABASE, DROP INDEX).

• TRUNCATE: Removes all records from a table quickly, but keeps the table structure intact.

• RENAME: Renames a database object.

Transaction Control Language (TCL)

TCL statements manage transactions within a database to ensure data integrity and reliability.

• COMMIT: Permanently saves the work done within the current transaction.

• ROLLBACK: Reverts the database to the state before the transaction began.

• SAVEPOINT: Sets a point within a transaction to which you can later roll back, without affecting previous work in the same transaction.

• BEGIN TRANSACTION/START TRANSACTION: Marks the beginning of a transaction.

Data Control Language (DCL)

DCL statements are used to control access and permissions to the database.

• GRANT: Gives user access privileges to the database.

• REVOKE: Removes access privileges from a user.

Other Significant Statements/Elements

• Aggregate Functions: COUNT(), SUM(), AVG(), MAX(), MIN() are used to perform calculations on a set of rows.

• Joins: INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN, CROSS JOIN are used to combine rows from multiple tables.

• Constraints: PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, CHECK, DEFAULT enforce data integrity.

• Stored Procedures/Functions: CREATE PROCEDURE, CREATE FUNCTION, EXECUTE PROCEDURE manage and run encapsulated logic.

• Performance Tuning: EXPLAIN (or EXPLAIN ANALYZE), ANALYZE TABLE, OPTIMIZE TABLE help in understanding and improving query performance.

Commands

ALTER TABLE

ALTER TABLE table_name

ADD column_name datatype;Copy to clipboard

ALTER TABLE lets you add columns to a table in a database.

AND

SELECT column_name(s)

FROM table_name

WHERE column_1 = value_1

AND column_2 = value_2;

AND is an operator that combines two conditions. Both conditions must be true for the row to be included in the result set.

AS

SELECT column_name AS 'Alias'

FROM table_name;

AS is a keyword in SQL that allows you to rename a column or table using an alias.

AVG()

SELECT AVG(column_name)

FROM table_name;

AVG() is an aggregate function that returns the average value for a numeric column.

BETWEEN

SELECT column_name(s)

FROM table_name

WHERE column_name BETWEEN value_1 AND value_2;

The BETWEEN operator is used to filter the result set within a certain range. The values can be numbers, text or dates.

CASE

SELECT column_name,

CASE

WHEN condition THEN 'Result_1'

WHEN condition THEN 'Result_2'

ELSE 'Result_3'

END

FROM table_name;

CASE statements are used to create different outputs (usually in the SELECT statement). It is SQL’s way of handling if-then logic.

COUNT()

SELECT COUNT(column_name)

FROM table_name;

COUNT() is a function that takes the name of a column as an argument and counts the number of rows where the column is not NULL.

CREATE TABLE

CREATE TABLE table_name (

column_1 datatype,

column_2 datatype,

column_3 datatype);

CREATE TABLE creates a new table in the database. It allows you to specify the name of the table and the name of each column in the table.

DELETE

DELETE FROM table_name

WHERE some_column = some_value;

DELETE statements are used to remove rows from a table.

GROUP BY

SELECT column_name, COUNT(*)

FROM table_name

GROUP BY column_name;

GROUP BY is a clause in SQL that is only used with aggregate functions. It is used in collaboration with the SELECT statement to arrange identical data into groups.

HAVING

SELECT column_name, COUNT(*)

FROM table_name

GROUP BY column_name

HAVING COUNT(*) > value;

HAVING was added to SQL because the WHERE keyword could not be used with aggregate functions.

INNER JOIN

SELECT column_name(s)

FROM table_1

JOIN table_2

ON table_1.column_name = table_2.column_name;

An inner join will combine rows from different tables if the join condition is true.

INSERT

INSERT INTO table_name (column_1, column_2, column_3)

VALUES (value_1, 'value_2', value_3);

INSERT statements are used to add a new row to a table.

IS NULL / IS NOT NULL

SELECT column_name(s)

FROM table_name

WHERE column_name IS NULL;

IS NULL and IS NOT NULL are operators used with the WHERE clause to test for empty values.

LIKE

SELECT column_name(s)

FROM table_name

WHERE column_name LIKE pattern;

LIKE is a special operator used with the WHERE clause to search for a specific pattern in a column.

LIMIT

SELECT column_name(s)

FROM table_name

LIMIT number;

LIMIT is a clause that lets you specify the maximum number of rows the result set will have.

MAX()

SELECT MAX(column_name)

FROM table_name;

MAX() is a function that takes the name of a column as an argument and returns the largest value in that column.

MIN()

SELECT MIN(column_name)

FROM table_name;

MIN() is a function that takes the name of a column as an argument and returns the smallest value in that column.

OR

SELECT column_name

FROM table_name

WHERE column_name = value_1

OR column_name = value_2;

OR is an operator that filters the result set to only include rows where either condition is true.

ORDER BY

SELECT column_name

FROM table_name

ORDER BY column_name ASC | DESC;

ORDER BY is a clause that indicates you want to sort the result set by a particular column either alphabetically or numerically.

OUTER JOIN

SELECT column_name(s)

FROM table_1

LEFT JOIN table_2

ON table_1.column_name = table_2.column_name;

An outer join will combine rows from different tables even if the join condition is not met. Every row in the left table is returned in the result set, and if the join condition is not met, then NULL values are used to fill in the columns from the right table.

ROUND()

SELECT ROUND(column_name, integer)

FROM table_name;

ROUND() is a function that takes a column name and an integer as arguments. It rounds the values in the column to the number of decimal places specified by the integer.

SELECT

SELECT column_name

FROM table_name;

SELECT statements are used to fetch data from a database. Every query will begin with SELECT.

SELECT DISTINCT

SELECT DISTINCT column_name

FROM table_name;

SELECT DISTINCT specifies that the statement is going to be a query that returns unique values in the specified column(s).

SUM

SELECT SUM(column_name)

FROM table_name;

SUM() is a function that takes the name of a column as an argument and returns the sum of all the values in that column.

UPDATE

UPDATE table_name

SET some_column = some_value

WHERE some_column = some_value;

UPDATE statements allow you to edit rows in a table.

WHERE

SELECT column_name(s)

FROM table_name

WHERE column_name operator value;

WHERE is a clause that indicates you want to filter the result set to include only rows where the following condition is true.

WITH

WITH temporary_name AS (

SELECT *

FROM table_name)

SELECT *

FROM temporary_name

WHERE column_name operator value;

WITH clause lets you store the result of a query in a temporary table using an alias. You can also define multiple temporary tables using a comma and with one instance of the WITH keyword.

Er. Pragati Pilaniya