SQL Server：知識點（1）

char(10) 定長 非Unicode 數據經常變的情況下使用，因爲不會導致數據頁更改
varchar(10) 變長 非Unicode
nchar(10) 定長 Unicode 適用於多國語言的情況
nvchar(10) 變長 Unicode

執行sql語句的順序
from>where>group by>having>select>order by

升序 order by asc
降序 order by desc
默認升序

case 表達式

select staff.id,
    case gender 
        when '男' then 'male'
    else 'female' end
from staff

取前幾條記錄

select top (3) with ties id from staff order by...
       top (10)percent

跳過第一條 取之後兩條

select id from staff order by id offset 1 rows fetch next 2 rows only

轉化
標準SQL cast
T-SQL convert

--代碼轉自互聯網

select CAST('123' as int)   -- 123
select CONVERT(int, '123')  -- 123

select CAST(123.4 as int)   -- 123
select CONVERT(int, 123.4)  -- 123 

select CAST('123.4' as int)
select CONVERT(int, '123.4')
-- Conversion failed when converting the varchar value '123.4' to data type int.

select CAST('123.4' as decimal)  -- 123
select CONVERT(decimal, '123.4') -- 123 


select CAST('123.4' as decimal(9,2))  -- 123.40
select CONVERT(decimal(9,2), '123.4') -- 123.40


declare @Num money
set @Num = 1234.56
select CONVERT(varchar(20), @Num, 0)  -- 1234.56
select CONVERT(varchar(20), @Num, 1)  -- 1,234.56
select CONVERT(varchar(20), @Num, 2)  -- 1234.5600