csharp-cheat-sheet/AdditionalNotes.md at master · oclipa/csharp-cheat-sheet

C#

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+ MVC

+ Model


TODO

TODO

+ View


TODO

TODO

+ Controller


TODO

TODO

+ GET vs POST


TODO

TODO

+ Request Flow


TODO

TODO

+ ViewState vs SessionState


TODO

TODO

+ XSD


TODO

TODO

+ XML Fragment vs XML Document


TODO

TODO

+ CORS


TODO

TODO

+ Git Interview Questions

+ What does `git reset` do?


TODO

+ What does `git push origin master` do?


TODO

+ HTML Interview Questions

+ What is the difference between Div and Span?


TODO

+ Why is it a good idea to split HTML and CSS?


TODO

+ What is the difference between Classes and Ids?


TODO

+ What is the difference between Margin and Padding?


TODO

+ To which elements can a hyperlink be applied?


TODO

+ What is the significant of a DocType?


TODO

+ JavaScript Interview Questions

+ What is the difference between Session Storage and Local Storage?


TODO

+ What does Strict mean?


TODO

+ What does NaN mean?


TODO

+ How do Timers work?


TODO

+ How can you enumerate data types?


TODO

+ How do you define Event Bubbling?


TODO

+ How do you define Event Bubbling?


TODO

+ What is the difference between TypeScript and JavaScript?


TODO

+ Is JavaScript Case Sensitive?


TODO

+ How does Object Inheritance work in JavaScript?


TODO

+ How does the `this` keyword work in JavaScript?


TODO

(also include notes from section on page 139, related to arrow functions)

+ What is Ajax?


TODO

+ Can you have private members in JavaScript?


TODO

(add notes from page 142, regarding true private methods)

+ What are some types of JavaScript errors?


TODO

+ How can you empty an array?


TODO

+ What is Function Hoisting?


TODO

+ What is the Deferred Attribute?


TODO

+ What is the Async Attribute?


TODO

+ What does `.innerHTML` do?


Inserts free-form HTML into the DOM, requiring a complete rebuild of the parent element.
Reserve for large DOM changes (e.g. long lists).

As shown in the following example, using .innerHTML can result in cleaner code, however it can be expensive to run.

myDiv.innerHTML += '<p class="paragraph" onclick="doSomething()">New Element</p>'

.innerHTML is best reserved for case where multiple changes must be made to the DOM (e.g. appending many items to a list):

let newElements = ''; 
for (i = 0; i < 100; i++) { 
 newElements += `<p>New Element Number: ${i}</p>` 
} 
myDiv.innerHTML =+ newElements;

+ What do `.createElement()` and `.appendChild()` do?
Add single elements to the DOM without requiring a complete rebuild.

Using .createElement() and .appendChild() is far more efficient when doing small updates to the DOM.

.createElement() is limited however, since it cannot add attributes to the element (e.g. classes or handlers).

let newElement = document.createElement('p'); 
newElement.textContent = 'New Element'; 
myDiv.appendChild(newElement);

If multiple updates must be made to the DOM (e.g. updating a long list), using .createElement() and .appendChild() can be much less efficient that .innerHTML:

for (i = 0; i < 100; i++) { 
 let newElement = document.createElement('p'); 
 newElement.textContent = 'New Element Number: ' + i;  
 div.appendChild(newElement); 
}

+ What does `.eval()` do?


It runs the interpreter, which is inefficient, and runs with elevated privileges, which can be dangerous.  Avoid.
If necessary, use `.Function()` instead, but other alternatives are preferable.

The main problem is that .eval() runs with the privileges of the caller, so in a server-side app, the caller could access features that are not permitted.

In client-side apps, .eval() is OK, although inefficient.

In server-side apps, it is a very bad idea unless you have complete control of what gets executed where.

An alternative is .Function(), which is both much faster and less insecure since it only runs in global scope (making it less likely to access something it shouldn't).

The best solution is to avoid whatever pattern is causing this type of function to be used.

For further information, see:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval

+ How can you hide code?


You can't!

Do not use the following:

<script language="javascript">
<!-- //
code here
//-->
</script>

+ SQL Interview Questions

+ What are common SQL commands?


SELECT, UPDATE, DELETE, INSERT INTO, CREATE DATABASE, ALTER DATABASE, CREATE TABLE, ALTER TABLE, DROP TABLE, CREATE INDEX, DROP INDEX

SELECT - extracts data from a database
UPDATE - updates data in a database
DELETE - deletes data from a database
INSERT INTO - inserts new data into a database
CREATE DATABASE - creates a new database
ALTER DATABASE - modifies a database
CREATE TABLE - creates a new table
ALTER TABLE - modifies a table
DROP TABLE - deletes a table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index

+ In what order should SQL commands be used?


     SELECT 
     FROM 
     WHERE 
     GROUP BY 
     HAVING 
     ORDER BY

WHERE is used to filter before GROUP BY

HAVING is used to filter after GROUP BY

+ What are the different types of Join?


(INNER) JOIN: Intersection only
LEFT (OUTER) JOIN: Intersection and Left only.
RIGHT (OUTER) JOIN: Intersection and Right only.
FULL (OUTER) JOIN : Everything.
SELF JOIN: Table is joined with itself.

Inner Join: "Select all rows from table1 and table2 where the column_name matches"

SELECT 
    column_name(s)
  FROM 
    table1
INNER JOIN 
    table2
  ON 
    table1.column_name = table2.column_name;

Left Join: "Select all rows from table1 plus those from table2 where the column_name matches a row in table1"

SELECT 
    column_name(s)
  FROM 
    table1
LEFT JOIN 
    table2
  ON 
    table1.column_name = table2.column_name;

Right Join: "Select all rows from table2 plus those from table1 where the column_name matches a row in table1"

SELECT 
    column_name(s)
  FROM 
    table1
RIGHT JOIN 
    table2
  ON 
    table1.column_name = table2.column_name;

Full Outer Join: "Select all rows from table1 or table2 where the column_name matches"

SELECT 
    column_name(s)
  FROM 
    table1
FULL OUTER JOIN 
    table2
  ON 
    table1.column_name = table2.column_name;

Self Join: "Select all rows from table1 where column_name1 does not match but column_name2 does match"

SELECT 
    column_name(s)
  FROM 
    table1 AS T1, // alias
    table1 AS T2  // alias
WHERE 
  T1.column_name1 <> T2.column_name1
AND 
  T1.column_name2 = T2.column_name2

+ What is the SQL Cursor?


Temporary work area containing select statement and rows to be acted on.
Effectively the same as a for/while loop (while has row, do something).

An SQL cursor is a temporary work area in memory where an SQL statement is executed. The cursor knows the select statement and the rows of data accessed.

The cursor can hold multiple rows but can only access one row at a time. The set of rows is called the Active Set.

The cursor effectively acts like a for/while loop, in that it can be used to iterate over a collection of rows.

A cursor is declared:

DECLARE 
  @person_id INT, 
  @name NVARCHAR(200);
DECLARE sample_cursor CURSOR 
FOR SELECT 
    person_id,
    name 
  FROM 
    company;

And used:

OPEN sample_cursor 
// get first row
FETCH NEXT FROM sample_cursor INTO 
  @person_id,
  @name; 
// while fetch is successful
WHILE @@FETCH_STATUS=0 
  BEGIN
    // do something
    
    // get next row
    FETCH NEXT FROM sample_cursor INTO 
      @person_id,
      @name;
  END; 
CLOSE sample_cursor; 
DEALLOCATE sample_cursor;

+ What is Index Fragmentation?


8KB pages of data no longer match logical order due to update actions.
Impact performance (Internal: unoptimized space; External: pages out of order).
5-30% = reorganize indexes.
>30% = rebuild indexes.
Base index keys on INSERT statements; don't update keys, don't use random keys; monitor database.

SQL databases have indexes that allow improved query performance. There are two types:

Clustered Index: one index per table; determines physical order of data in table (rows are stored physically on disk in this order)
Non-Clustered Index: multiple indexes per table; essentially pointers to physical rows (additional indexes decrease performance)

In both cases, an index points to pages, which are the smallest unit of data in the database, with each page consisting of 8KB of data (for legacy reasons related to HDD disk block sizes). In addition to data, each page contains pointers to the next and previous pages in the logical order.

Initially, the pages of data in a table will match the logical order of its sort keys and space will be optimized, however as actions are performed on the database (INSERT, UPDATE, DELETE) the pages will need to be split and moved around to accomodate new data. This is what is termed Index Fragmentation.

There are two types of Index Fragmentation:

Internal: pages gain empty space as data is moved between pages, which increases the number of logical reads required.
External: the order of the pages no longer matches their logical sort order, which increases the number of physical reads required.

If fragmention is between 5-30%, the indexes should be reorganized (fix external fragmentation by re-ordering pages).

If fragmention is greater than 30%, the indexes should be rebuilt (fix both internal and external fragmentation by re-creating all pages from scratch).

To reduce fragmentation:

Choose index keys that suit most common INSERT statements.
Don't update index key columns.
Don't choose random key values.
Monitor and maintain the database.

Move along; nothing to see here...

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