# Data Retrieval Commands

## `SELEC FROM Command`

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### Can I use select without from ?

1. Yes, using DUAL Tables.&#x20;
2. &#x20;Dual tables are dummy tables created by MySQL, help users to do certain obvious actions without referring to user

In SQL, you can use the `SELECT` statement without specifying a `FROM` clause, but it serves a different purpose when used this way. When you use `SELECT` without `FROM`, you are typically performing a query that doesn't involve selecting data from a database table but instead involves evaluating expressions or calculations.

Here are a few common scenarios where you might use `SELECT` without `FROM`:

1. **Selecting Constants or Expressions:** You can use `SELECT` to select constant values or calculate expressions without involving database tables. For example:

   ```sql
   SELECT 42; -- Selects the constant value 42
   SELECT 5 + 7; -- Selects the result of the expression 5 + 7 (12)
   ```
2. **Using Built-in Functions:** You can use `SELECT` to call built-in SQL functions and retrieve their results:

   ```sql
   SELECT CURRENT_DATE; -- Selects the current date
   SELECT UPPER('hello'); -- Selects 'HELLO'
   ```
3. **Aliasing:** You can also use `SELECT` without `FROM` to assign aliases to expressions or constants, which can be useful for making the query results more readable:

   ```sql
   SELECT 42 AS "Answer to Life, the Universe, and Everything";
   ```
4. **Subqueries:** You can use subqueries within a `SELECT` statement without specifying a `FROM` clause. These subqueries can return single values or result sets that are used in the main query.

   ```sql
   SELECT (SELECT MAX(salary) FROM employees) AS max_salary;
   ```

In summary, while the primary use of the `SELECT` statement is to retrieve data from database tables using the `FROM` clause, you can also use it without `FROM` for various other purposes, such as selecting constants, performing calculations, using functions, aliasing, and working with subqueries.

## `WHERE CLAUSE`

The `WHERE` clause in SQL is used to filter rows from a table based on a specified condition. It allows you to retrieve only the rows that meet the specified criteria, excluding those that do not. The `WHERE` clause is commonly used in `SELECT`, `UPDATE`, and `DELETE` statements.

```sql
SELECT * FROM `worker` WHERE salary > 55000;
```

```sql
SELECT * FROM `worker` WHERE manager_id = 1;
```

```sql
SELECT * FROM `worker` WHERE department = "Engineering";
```

## `BETWEEN`

`[ RANGE1 , RANGE2 ] INCLUDED`

```sql
SELECT * FROM `worker` WHERE salary BETWEEN 0 AND 55000;
```

## `IN`

Reduces OR conditions;

```sql
SELECT * FROM `worker` WHERE department IN ('HR' , 'Account');
```

## `AND/OR/NOT`

1. AND: WHERE cond1 AND cond2
2. OR: WHERE cond1 OR cond2
3. NOT: WHERE col\_name NOT IN (1,2,3,4);

```sql
SELECT * FROM `worker` WHERE department NOT IN ('HR' , 'Account');
```