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The SQL CASE Statement

The CASE statement goes through conditions and returns a value when the first condition is met (like an if-then-else statement). So, once a condition is true, it will stop reading and return the result. If no conditions are true, it returns the value in the ELSE clause.
If there is no ELSE part and no conditions are true, it returns NULL.

CASE Syntax

CASE
WHEN condition1 THEN result1
WHEN condition2 THEN result2
WHEN conditionN THEN resultN
ELSE result
END;


Agar bitta case ga tushsa boshqasiga kirmaydi va keyingi rowga o’tadi.


Isnull function isnull(expression, alternative)
Expression – test uchun qiymat
Altervative – agar null bo’lsa qaytarishligi kerak bo’lgan qiymat.
expression agar null bo’lmasa isnull expressionni o’zini qaytaradi.

The key difference between varchar and nvarchar is the way they are stored, varchar is stored as regular 8-bit data(1 byte per character) and nvarchar stores data at 2 bytes per character. Due to this reason, nvarchar can hold upto 4000 characters and it takes double the space as SQL varchar

SQL Comments

Comments are used to explain sections of SQL statements, or to prevent execution of SQL statements.
Note: The examples in this chapter will not work in Firefox and Microsoft Edge!
Comments are not supported in Microsoft Access databases. Firefox and Microsoft Edge are using Microsoft Access database in our examples.

Single Line Comments

Single line comments start with --.
Any text between -- and the end of the line will be ignored (will not be executed).
The following example uses a single-line comment as an explanation:

Example

--Select all:
SELECT * FROM Customers;