Converting Column into Rows In SQL Server, Sql Server

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create table t (child_code varchar(6), plan_date datetime);
    insert into t values ('000001','20170221'),('000001','20170321');
    declare @cols nvarchar(max);
    declare @sql  nvarchar(max);
    
      select @cols = stuff((
        select distinct 
          ',' + quotename('Plan_Date_'
              +convert(nvarchar(10),row_number() over (
                  partition by Child_Code 
                  order by     Plan_Date 
              ))
              )
          from t 
          for xml path (''), type).value('.','nvarchar(max)')
        ,1,1,'');
        
    select @sql = '
     select Child_Code, ' + @cols + '
     from  (
      select 
          Child_Code
        , Plan_Date
        , rn=''Plan_Date_''+convert(nvarchar(10),row_number() over (
              partition by Child_Code 
              order by     Plan_Date 
            ))
      from t
        ) as a
     pivot (max([Plan_Date]) for [rn] in (' + @cols + ') ) p';
     select @sql as CodeGenerated;
     exec sp_executesql @sql;

View schema
Execution time: 0,02 sec, rows selected: 4, rows affected: 2, absolute service time: 0,16 sec