Update sequence in SQL Server

Question

Create Script :

CREATE TABLE [dbo].[tblTEST]
(
    [AccountNO] [varchar](10) NOT NULL,
    [Serial] [int] NOT NULL,
    [AccountType] [varchar](1) NOT NULL,
         


        

Answer 1

Steps of update statement execution is

1. retrieval from table and variable assignment from left side
2. set to data to table from the left site

so the sequence of execution of your update statement will be as follows-

A.Balance = @Balance --4
, @PreBalance = @Balance --1
, @Balance = ( CASE WHEN @Balance IS NULL OR @AccountType <> A.AccountType
                    THEN A.Balance
                    ELSE @Balance - A.Due
                END ) --2
, A.Flag = CASE WHEN @PreBalance = A.Balance THEN 0 ELSE 1 END --5
, @AccountType = A.AccountType --3

example

AccountNO Serial AccountType Due       Balance   Flag 
--------- ------ ----------- --------- --------- ---- 
A1        1      1           1000.0000 2000.0000 0    
A1        2      1           1000.0000 2000.0000 0   

execution will be

row 1 phase 1

, @PreBalance = null --1
, @Balance = ( CASE WHEN null IS NULL OR null <> A.AccountType
                    THEN A.Balance
                    ELSE null - A.Due
                END )  --2
, @AccountType = A.AccountType --3

row 1 phase 2

A.Balance = 2000 --4
, A.Flag = CASE WHEN null = A.Balance THEN 0 ELSE 1 END --5

row 2 phase 1

, @PreBalance = 2000 --1
, @Balance = ( CASE WHEN 2000 IS NULL OR 1 <> A.AccountType
                    THEN A.Balance
                    ELSE 2000 - A.Due
                END ) --2
, @AccountType = A.AccountType --3

row 2 phase 2

A.Balance = 1000 --4
, A.Flag = CASE WHEN 2000 = 1000 THEN 0 ELSE 1 END --5

so on