In PostgreSQL, the `UPPER` and `LOWER` functions are used to manipulate the case of text strings. Here's a quick explanation of each function with an example:

UPPER Function:
The UPPER function converts all characters in a text string to uppercase.

Syntax:

UPPER(string) 

 
LOWER Function:
The LOWER function converts all characters in a text string to lowercase.

Syntax:

LOWER(string) 

In PostgreSQL, a view is a virtual table created by a query. It behaves like a table but doesn't store data itself; instead, it's a saved query that can be referenced like a table. Views are useful for simplifying complex queries, providing an abstracted layer over tables, and controlling access to data.

Explanation:
1. A view is defined by a SQL query and has a name.
2. The view's data is dynamically generated based on the query whenever the view is queried.
3. Views can simplify complex joins or aggregations and abstract the underlying data structure for users.

 
Example:
Suppose you have a database with a table orders containing order information with columns like `order_id`, `customer_id`, and `order_date`. You want to create a view that shows only the orders placed by a specific customer. Here's how you can do it:

-- Create a view named "customer_orders" to show orders for a specific customer 
CREATE VIEW customer_orders AS 
SELECT order_id, order_date 
FROM orders 
WHERE customer_id = '12345';

In PostgreSQL, the SUBSTRING function is used to extract a portion of a string from within another string. It allows you to specify the starting position and the length of the substring you want to extract. 
 
Example:

SUBSTRING(string FROM start_position [FOR length])

In PostgreSQL and SQL in general, the `WHERE` clause, combined with the `IN` operator, is used to filter rows from a table based on a specified list of values. It allows you to retrieve rows that match any of the values provided in the list. The `IN` operator simplifies querying for multiple values without the need for multiple `OR` conditions.

Example:

SELECT column1, column2, ... 
FROM table_name 
WHERE column_name IN (value1, value2, ...);

In JavaScript, the double exclamation mark (!!) is often used as a shorthand to convert a value to its corresponding Boolean representation. It essentially coerces a value to either true or false. Here's a quick explanation and example of its use:
 
Example:

let someValue = "Hello";
let isTruthy = !!someValue; 
console.log(isTruthy); // truesomeValue = 0;
let isFalsy = !!someValue; 
console.log(isFalsy); // false 

"git grep" is a Git command used for searching through your Git repository's files for specific text patterns. It's a powerful tool for finding where a particular string or regular expression appears in your codebase. The "git grep" command can be especially helpful for debugging, refactoring, or finding specific references in your code. 

git grep [options] pattern