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Complete SQL Reference's

Intro Of SQL

SQL (pronounced "ess-cue-ell" or sometimes "sequel") stands for Structured Query Language. It is the standard language used to communicate with relational databases — databases that store data in tables with rows and columns.

📘 SQL Keywords
SELECT	Used to select data from a database
FROM	Specifies the table to select or delete data from
WHERE	Filters records
INSERT INTO	Adds new data into a table
VALUES	Provides values for an INSERT
UPDATE	Modifies existing data
SET	Defines new values in UPDATE
DELETE	Deletes data from a table
CREATE TABLE	Creates a new table
ALTER TABLE	Modifies a table structure
DROP TABLE	Deletes a table
JOIN	Combines rows from multiple tables
INNER JOIN	Returns rows with matching values
LEFT JOIN	All rows from left + matched from right
RIGHT JOIN	All rows from right + matched from left
FULL JOIN	All rows from both tables
GROUP BY	Groups rows sharing a property
ORDER BY	Sorts results
HAVING	Filters grouped records
AS	Alias for a column or table
DISTINCT	Returns only distinct values
LIMIT	Specifies number of records to return
OFFSET	Skips number of records
IN	Matches any in a list
NOT IN	Excludes from list
LIKE	Pattern matching
IS NULL	Checks for NULL
UNION	Combines multiple SELECT results
CASE	Conditional logic

📦 SQL Data Types
INT	Integer (whole number)
FLOAT / DOUBLE	Floating point numbers
DECIMAL(p,s)	Exact numeric value with precision
CHAR(n)	Fixed-length character
VARCHAR(n)	Variable-length character
TEXT	Large text
DATE	YYYY-MM-DD format
TIME	HH:MM:SS format
DATETIME	Date and time
BOOLEAN	TRUE/FALSE values

📌 SQL Constraints
PRIMARY KEY	Uniquely identifies a row
FOREIGN KEY	Links to another table's primary key
UNIQUE	Ensures unique values
NOT NULL	Value cannot be null
DEFAULT	Sets a default value
CHECK	Ensures condition is true
AUTO_INCREMENT	Auto-increments numeric field

⚙️ SQL Functions
COUNT()	Returns number of rows
SUM()	Returns the total sum
AVG()	Returns the average
MIN()	Returns the minimum value
MAX()	Returns the maximum value
NOW()	Returns current datetime
UPPER()	Converts to uppercase
LOWER()	Converts to lowercase
ROUND()	Rounds a number
LEN() / LENGTH()	Length of a string
SUBSTRING()	Extract part of string

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