SQL--NOtes

 

🔵 SQL Learning Notes

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🟦 Day 1 – Introduction to SQL

🟢 What is SQL?

SQL (Structured Query Language) is a language used to communicate with relational databases.

🟨 Uses of SQL

• Query Data

• Update Data

• Delete Data

• Alter Data

🟨 Important Terminology

Term	Also Known As
Column	Field, Attribute
Row	Record, Tuple

🟨 Schema vs Database

✅ MySQL

• Schema and Database are the same.

✅ MS SQL Server & PostgreSQL

• Schema acts as a namespace within a database.

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🟦 Day 2 – SQL Query Execution Order

🟨 Execution Order

1. FROM and JOIN

2. WHERE

3. GROUP BY

4. HAVING

5. ORDER BY

6. SELECT

7. TOP

🟨 Important Concepts

Aggregated Values

Non-Aggregated Values

DISTINCT = Unique Values

ASC = Lowest to Highest ⬆️

DESC = Highest to Lowest ⬇️

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🟦 Day 3 – SQL Commands

🟢 DQL (Data Query Language)

SELECT

🟠 DDL (Data Definition Language)

CREATE
DROP
ALTER

🟣 DML (Data Manipulation Language)

INSERT
UPDATE
DELETE

⭐ Shortcuts

🟣 DML = IUD

🟠 DDL = CDA

🟨 Best Practice (INSERT)

INSERT INTO schema_name.table_name
(column1, column2, column3)
VALUES
(value1, value2, value3);

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🟦 Day 4 – INSERT, UPDATE & DELETE

🟢 INSERT

MySQL

INSERT INTO tbl (col1, col2)
VALUES (val1, val2);

Auto ID: AUTO_INCREMENT

SQL Server

INSERT INTO tbl (col1, col2)
VALUES (val1, val2);

Auto ID: IDENTITY

PostgreSQL

INSERT INTO tbl (col1, col2)
VALUES (val1, val2);

Auto ID:

• SERIAL

• GENERATED AS IDENTITY

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🟠 UPDATE

MySQL

UPDATE tbl
SET col1 = val1
WHERE condition;

SQL Server

UPDATE tbl
SET col1 = val1
WHERE condition;

PostgreSQL

UPDATE tbl
SET col1 = val1;

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🔴 DELETE

MySQL

DELETE FROM tbl
WHERE condition;

SQL Server

DELETE FROM tbl
WHERE condition;

PostgreSQL

DELETE FROM tbl
WHERE condition;

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⭐ Today's Learning

🟨 Key Points

1. INSERT INTO (COLUMN NAME WITHOUT '')

2. AUTO_INCREMENT OR SERIAL BOTH WORK IN MYSQL

3. AFTER TABLE CREATION AUTO_INCREMENT = VALUE SET

4. INT IDENTITY(SEED, INCREMENT) IN MS SQL

5. SERIAL OR INT GENERATED ALWAYS AS IDENTITY

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🟦 ALTER TABLE

🟢 Add Column

MS SQL

ALTER TABLE employess
ADD email VARCHAR(50);

MySQL & PostgreSQL

ALTER TABLE employess
ADD COLUMN email VARCHAR(50);

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🟢 Change Data Type

PostgreSQL

ALTER TABLE employess
ALTER COLUMN salary TYPE INTEGER
USING salary::INTEGER;

MySQL

ALTER TABLE employess
MODIFY COLUMN salary INTEGER;

MS SQL

ALTER TABLE employess
ALTER COLUMN salary INT;

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🟦 UPDATE Examples

Update Specific Record

UPDATE users
SET age = 31,
    city = 'kolkat1a'
WHERE username = 'Priya';

Increase Age by 1

UPDATE users
SET age = age + 1
WHERE email LIKE '%@gmail.com';

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🔴 DELETE Example

DELETE FROM users
WHERE user_id = 6;

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🟦 Rename Column

PostgreSQL & MySQL

ALTER TABLE users
RENAME COLUMN username TO full_name;

MS SQL

EXEC sp_rename 'users.username', 'full_name', 'COLUMN';

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🟦 ORDER BY

SELECT *
FROM users
ORDER BY user_id ASC;

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🟦 Change Data Type

PostgreSQL

ALTER TABLE users
ALTER COLUMN age TYPE SMALLINT;

MySQL & MS SQL

ALTER TABLE users
ALTER COLUMN age SMALLINT;

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🟦 Add NOT NULL Constraint

PostgreSQL

ALTER TABLE users
ALTER COLUMN city SET NOT NULL;

MS SQL

ALTER TABLE users
ALTER COLUMN city VARCHAR(50) NOT NULL;

MySQL

ALTER TABLE users
MODIFY COLUMN city VARCHAR(50) NOT NULL;

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🟦 CHECK Constraint

Drop Constraint

ALTER TABLE users
DROP CONSTRAINT age;

Add Constraint

ALTER TABLE users
ADD CONSTRAINT age
CHECK(age >= 18);

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🚀 Quick Revision

🟢 DQL

SELECT

🟠 DDL

CREATE
DROP
ALTER

🟣 DML

INSERT
UPDATE
DELETE

🟨 Execution Order

FROM
WHERE
GROUP BY
HAVING
ORDER BY
SELECT
TOP

⬆️ ASC

Lowest to Highest

⬇️ DESC

Highest to Lowest

⭐ Shortcuts

DML = IUD
DDL = CDA
DISTINCT = UNIQUE

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