Type of Loops in SQL Server

Syntax of for-loop in SQL Server

T-SQL doesn't have a FOR loop, it has a WHILE loop

WHILE (Transact-SQL)

WHILE Boolean_expression
BEGIN

END

How to loop a query in different databases in T-SQL?

I like creating a temp table for the query results, creating a table variable for my list of dbs to query, and then looping through that list of databases to execute the query for each, inserting the results into the temp table.

This way you can query the results repeatedly after running the whole loop process only once.

Try this (you'll need to modify the #results temp table declaration and the where clause of the first insert statement):

--drop temp table if it exists; this clears the results each time you run the full script
if object_id('tempdb..#results') is not null
    drop table #results
go

--recreate temp table; this is used for storing the result set.  It's created as a temp table instead of a table variable so that it can be queried repeatedly after gathering results.
create table #results (
    dbName varchar(128)
    ,cust_num varchar(128)
    , [name] varchar(128)
    , credit_hold bit
    , credit_hold_reason varchar(128)
    , [Type of credit hold reason] varchar(128)
    , credit_hold_date varchar(128)
    , credit_limit int);  --adjust column declaration as needed for query results

--create a variable to track the database list
declare @dbList table (dbName varchar(128), indx int)

insert into @dbList
select dbName = name, row_number() over (order by name) 
from master.sys.databases
--where --insert your own where clause here to narrow down which databases to run the query on.

--declare variables for use in the while loop
declare @index int = 1
declare @totalDBs int = (select count(*) from @dbList)
declare @currentDB varchar(128)
declare @cmd varchar(300)
--define the command which will be used on each database.
declare @cmdTemplate varchar(300) = '
    use {dbName};
    insert into #results
    select db_name(), cust_num, name, credit_hold, credit_hold_reason, 
       (case 
           when credit_hold_reason = ''NP'' then ''No payments''
           when credit_hold_reason = ''UK'' then ''Unknown''
           when credit_hold_reason = ''BK'' then ''Bankruptcy''
        end) as ''Type of credit hold reason'',
     credit_hold_date, credit_limit
    from [custaddr] 
    order by credit_hold_reason asc
'

--loop through each database and execute the command
while @index <= @totalDBs 
begin
    set @currentDB = (select dbName from @dbList where indx = @index)
    set @cmd = replace(@cmdTemplate, '{dbName}', @currentDB)

    execute(@cmd)

    set @index += 1
end

--see the results of the command; this can be queried repeatedly by itself after it's created the first time
select *
from #results

Note that looping through a table variable is more efficient than declaring a cursor.

Looping through all tables and then looping through all column datatypes

You can build the statements in a string and execute it dynamically. That way you don't have a need for while-loops.

DECLARE @alter_stmts NVARCHAR(MAX) = (
    SELECT 
        ';ALTER TABLE '+
            QUOTENAME(t.TABLE_SCHEMA)+'.'+QUOTENAME(t.TABLE_NAME)+' '+
        'ALTER COLUMN '+
            QUOTENAME(c.COLUMN_NAME)+ ' NVARCHAR(MAX) ' + CASE WHEN c.IS_NULLABLE='YES' THEN 'NULL' ELSE 'NOT NULL' END 
    FROM 
        INFORMATION_SCHEMA.TABLES AS t
        INNER JOIN INFORMATION_SCHEMA.COLUMNS AS c ON
            c.TABLE_SCHEMA=t.TABLE_SCHEMA AND
            c.TABLE_NAME=t.TABLE_NAME
    WHERE
        t.TABLE_TYPE='BASE TABLE' AND
        c.DATA_TYPE LIKE '%NTEXT%'
    FOR
        XML PATH('')
)+';';

--SELECT @alter_stmts; -- review
EXEC sp_executesql @alter_stmts; -- execute it

Why is the same value being return in this SQL While Loop?

The issue happens because SQL Server doesn't associate @row with a row in the table (the correlation is obvious to you, but SQL Server isn't human).

As you loop through the numbers 1 -> @count, it is running the same SELECT @ABC = ABC FROM #TMP_ABC over and over again. There is no WHERE clause and no TOP so SQL Server is just reading the whole table every time, and setting the variable equal to the last ABC value it read.

Instead, you should use a cursor (if you need to loop at all; usually you don't, per @Larnu's comment). You have have read some misinformation somewhere that cursors are bad and that while loops are not cursors, but these are both false.

• Bad Habits to Kick : Thinking a WHILE loop isn't a CURSOR
• What impact can different cursor options have?
• Follow-up on cursor options
• Overlooked T-SQL Gems (see why using a local variable for a cursor is even better than the regular type you probably use)

If you do in fact need to loop for some reason, here's a rewrite:

CREATE TABLE #TMP_ABC(ABC varchar(3));

INSERT INTO #TMP_ABC(ABC) VALUES 
  ('AAA'),('BBB'),('CCC'),('DDD'),('EEE'),('FFF');
  
DECLARE @ABC varchar(3), @c cursor;
    
SET @c = cursor LOCAL FAST_FORWARD
    FOR SELECT ABC FROM #TMP_ABC;

OPEN @c;
FETCH NEXT FROM @c INTO @ABC;

WHILE @@FETCH_STATUS = 0
BEGIN
  PRINT @ABC;
  FETCH NEXT FROM @c INTO @ABC;
END

Output:

AAA
BBB
CCC
DDD
EEE
FFF

• Example db<>fiddle

But I'm not sure what that accomplishes over SELECT ABC FROM #TMP_ABC;.

Do while loop in SQL Server 2008

I am not sure about DO-WHILE IN MS SQL Server 2008 but you can change your WHILE loop logic, so as to USE like DO-WHILE loop.

Examples are taken from here: http://blog.sqlauthority.com/2007/10/24/sql-server-simple-example-of-while-loop-with-continue-and-break-keywords/

1. Example of WHILE Loop

   DECLARE @intFlag INT
   SET @intFlag = 1
   WHILE (@intFlag <=5)
   BEGIN
       PRINT @intFlag
       SET @intFlag = @intFlag + 1
   END
   GO
   

   
   

   ResultSet:

   1
   2
   3
   4
   5
   

2. Example of WHILE Loop with BREAK keyword

   DECLARE @intFlag INT
   SET @intFlag = 1
   WHILE (@intFlag <=5)
   BEGIN
       PRINT @intFlag
       SET @intFlag = @intFlag + 1
       IF @intFlag = 4
           BREAK;
   END
   GO
   

   
   

   ResultSet:

   1
   2
   3
   

3. Example of WHILE Loop with CONTINUE and BREAK keywords

   DECLARE @intFlag INT
   SET @intFlag = 1
   WHILE (@intFlag <=5)
   BEGIN
       PRINT @intFlag
       SET @intFlag = @intFlag + 1
       CONTINUE;
       IF @intFlag = 4 -- This will never executed
           BREAK;
   END
   GO
   

   
   

   ResultSet:

   1
   2
   3
   4
   5
   

But try to avoid loops at database level.
Reference.

SQL fill next date (month) with loop

Paul, I'm assuming that you forgot to specify the month in your mock data.

I hope the code below may help you understand how non-trivial is what you are trying to accomplish :-) Kudos for your will to get rid of loops.

To make it better, I propose a denormalization (CAUTION!):

• create another column price_valid_until
• the latest prices records will have price_valid_until = '21000101' (aka, far away in the future)
• when registering a new price, update the previous with new price_valid_from - 1 day

Here's the solution, with a pretty complex, but efficient query (http://sqlfiddle.com/#!18/4ab23/4)

create table price_history(
  SKU varchar(255),
  price_valid_from date,
  price decimal(16, 2)
)

insert into price_history
values
  ('a', '20210101', 10),
  ('a', '20210107', 12),
  ('b', '20210102', 4),
  ('b', '20210110', 2),
  ('b', '20210214', 5);

-- This fiddler won't let me initialize and reference:
-- 
-- declare
--   @from_date date,
--   @to_date   date;
-- 
-- select
--   @from_date = min(date_from),
--   @to_date   = max(date_from)
-- from price_history

with
  date_range as(
    select
      min(price_valid_from) as from_date,
      --
      eomonth(
        max(price_valid_from)
      ) as to_date
    from price_history
  ),
  --
  all_dates as(
    select from_date as date_in_range
    from date_range
    -- ----------
    union all
    -- ----------
    select dateadd(day, 1, date_in_range)
    from all_dates
    where
      date_in_range < (
        select to_date
        from date_range
      )
  ),
  --
  price_history_boundaries as(
    select
      ph.SKU,
      ph.price,
      --
      ph.price_valid_from,
      -- The latest price, so far, is valid until 01/01/2100
      coalesce(
        dateadd(
          day,
          -1,
          min(ph_next.price_valid_from)
        ),
        '21000101'
      ) as price_valid_until
    from
      price_history ph
        left outer join price_history ph_next
        on(
              ph_next.SKU              = ph.SKU
          and ph_next.price_valid_from > ph.price_valid_from
        )
    group by ph.SKU, ph.price_valid_from, ph.price
  )
select
  phb.SKU,
  ad.date_in_range,
  phb.price
from
  all_dates ad
    inner join price_history_boundaries phb
    on(
          phb.price_valid_from  <= ad.date_in_range
      and phb.price_valid_until >= ad.date_in_range
    )
order by phb.SKU, ad.date_in_range

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