T-SQL intro

Quick overview of T-SQL and database with supporting Map

### The Essential T-SQL Clauses and the ACID Principles Explained

T-SQL (Transact-SQL) is an extension of SQL (Structured Query Language) used to interact with SQL Server databases. It includes several key clauses that help in querying and managing data. Here's a look at the five essential T-SQL clauses and an explanation of the ACID principles, which are fundamental to database transactions.

#### 1. SELECT Clause

The `SELECT` clause is the backbone of any SQL query. It is used to retrieve data from one or more tables. With `SELECT`, you can specify the columns you want to fetch and apply various filtering and sorting options.

```sql

SELECT column1, column2

FROM table_name

WHERE condition

ORDER BY column1;

```

#### 2. FROM Clause

The `FROM` clause specifies the table(s) from which to retrieve the data. It is used in conjunction with the `SELECT` clause to define the source of your data.

```sql

SELECT *

FROM table_name;

```

#### 3. WHERE Clause

The `WHERE` clause is used to filter records that fulfill a specified condition. It helps in narrowing down the results to only those rows that meet the criteria.

```sql

SELECT *

FROM table_name

WHERE condition;

```

#### 4. JOIN Clause

The `JOIN` clause is used to combine rows from two or more tables, based on a related column between them. Different types of joins (INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN) help in fetching related data across multiple tables.

```sql

SELECT table1.column1, table2.column2

FROM table1

JOIN table2

ON table1.common_column = table2.common_column;

```

#### 5. ORDER BY Clause

The `ORDER BY` clause is used to sort the result set in either ascending or descending order. By default, it sorts the records in ascending order.

```sql

SELECT *

FROM table_name

ORDER BY column1 ASC, column2 DESC;

```

### Understanding the ACID Principles

ACID stands for Atomicity, Consistency, Isolation, and Durability. These principles ensure that database transactions are processed reliably.

#### Atomicity

- **Atomicity** guarantees that each transaction is treated as a single unit, which either completely succeeds or completely fails. If any part of the transaction fails, the entire transaction is rolled back, and the database state remains unchanged.

#### Consistency

- **Consistency** ensures that a transaction brings the database from one valid state to another, maintaining database invariants. This means that any transaction will leave the database in a consistent state, maintaining all predefined rules, such as constraints, cascades, and triggers.

#### Isolation

- **Isolation** ensures that transactions occurring concurrently do not affect each other's outcome. The operations within a transaction are isolated from those in other transactions, preventing the effects of one transaction from being visible to others until it is completed.

#### Durability

- **Durability** ensures that once a transaction has been committed, it remains committed, even in the event of a system failure. This means that the results of the transaction are permanently recorded in the database.

By understanding these T-SQL clauses and the ACID principles, you can effectively manage and interact with SQL Server databases, ensuring reliable and efficient data transactions.

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