SQL Crib Sheet for iSeries and .NET

A friend was asking me for some SQL samples the other day so I dug up this old crib sheet from some time ago. These can be used with slight modification in both iSeries and .NET queries. Enjoy!

Select Statements

Return all records all columns in a table:

    select * from MyTableq3

Return all records but only cqcus and cqcush in a table:

    select cqcus, cqcush from MyTableq3

Return cqcus for all records in a table with a specific value for cqcush:

    select cqcus from MyTableq3 where cqcush=123

Return all records in a table where cqcus is one of three possible values:

    select * from MyTableq3 where cqcus in (value1,value2,value3)

Return the number of records in a table:

    select count(*) from TheTable

Return the number of records in a table with a specific value for cqcush:

    select count(*) from MyTableq3 where cqcush=123

Simple join:

    select * from MyTableq3, MyTable
    where MyTableq3.cqcus=MyTable .cqcusA

or

    select MyTableq3.cqcus, MyTable .cqcusA from MyTableq3, MyTable
    where MyTableq3.cqcush=MyTable .cqcusB

Select all unique values in cqcus from a table:

    select distinct(cqcus) from MyTableq3

or

    select distinct cqcus from MyTableq3

Select all unique values for cqcus from a table together with the number of records with that unique value:

    select cqcus, count(*) from MyTableq3
    group by cqcus

Select all unique values for combinations of cqcus and cqcush from a table together with the number of records with that combination:

    select cqcus, cqcush, count(*) from MyTableq3
    group by cqcus, cqcush

Select the number of unique values:

    select count(distinct cqcus) from MyTableq3

Select all duplicate records in a table, where two (or more) records are considered duplicates if they share a common value for a single cqcus:

    select cqcus, count(cqcus) from MyTableq3
    group by cqcus
    having count(*) > 1

Select all duplicate records in a table, where two (or more) records are considered duplicates if they share common values for a pair of cqcuss:

    select cqcus, cqcush, count(*) from MyTableq3
    group by cqcus, cqcush
    having count(*) > 1

Select similar records, i.e. all records which have duplicate cqcus and cqcush in a table but with different cqcus3 (i.e. specifying which cqcuss must be the same and which different):

    select * from table as A, table as B
    where A.cqcus=B.cqcus
    and A.cqcush=B.cqcush
    and A.cqcus3<>B.cqcus3;

Note:

    * It is important to specify at least one cqcus which is different between the two records otherwise this query will list a record as being the same as itself.
    * This query will not find duplicate records, i.e. records with every cqcus the same.

Select all records from a table which do not share a common ID with records from a second table:

    select * from MyTableq3
    where cqcus not in (select cqcush from MyTable )

Note:

    * Sub-queries are quite slow.
    .

An alternative using a join (which can be much faster):

    select MyTableq3.* from MyTableq3
    left join MyTable  on (MyTableq3.cqcus = MyTable .cqcush)
    where MyTable .cqcush is null;

The following method (which has been suggested by Michael Miller) is to use EXISTS. It is much faster on SQL Server than the above (but Michael says it is comparable with the left join technique on Oracle):

    select * from MyTableq3
    where not exists (select cqcush from MyTable  where MyTable .cqcush = MyTableq3.cqcus)

To perform a two way join:

    select * from
    MyTableq3 left join MyTable  on (MyTableq3.cqcus = MyTable .cqcus),
    MyTableq3 left join table3 on (MyTableq3.cqcush = table3.cqcus3)

this has been tested on SQL Server, but not on Oracle or MySql. It does not work with MS-Access.

To combine the results of two queries (be aware that the number and types of cqcuss in both queries must agree):

    select * from MyTableq3
    union select * from MyTable

To return a value based on the contents of a cqcus. This can be done using either Iif, Decode or Case, depending on the database.

The following works with MSAccess:

    select Iif(cqcus = 1, 'one', 'not one')
    from MyTableq3

This is equivalent to the following on SqlServer:

    select Case when cqcus = 1 then 'One' else 'Two' End

    from MyTableq3

Direct join to see if any records from table 1 match table 2
SELECT statement run complete.          
  select * from MyTableq3, MyTable        
    where cqcus=cfcus and  cqcush=cfcush
and cqvar = cfvar                      
and cqlv = cflv                        
and cqsizgrp = cfsizgrp                

For Oracle use the DECODE function.

To create a new table to hold the results of the select query:

    select * into MyTable  from MyTableq3

Be aware that this will fail if MyTable  exists, and that the new table will be created without any indexes.
Insert

Insert new record into a table:

    insert into MyTableq3 values (1,2,3)

Insert new record into a table explicitly naming cqcuss:

    insert into MyTableq3 (cqcus,cqcush,cqcus3) values (1,2,3)

Insert new record into a table using values from another table:

    insert into MyTableq3 (cqcus,cqcush,cqcus3)
    select cqcusA,2,cqcusC from SomeTable

Update

Update all records in a table:

    update MyTableq3 set cqcus=2

Update specific records in a table:

    update MyTableq3 set cqcus=2 where cqcus=1

To update more than one cqcus at a time:

    update MyTableq3 set cqcus=2, cqcush=3

Update a cqcus in a table using a value from another table where both records are referenced by a common key - warning, different databases support different syntax!

    This works in MS-Access and MySQL (5) but not in SQL Server:

    update TableOne
        inner join TableTwo on TableOne.commonID = TableTwo.commonID
        set TableOne.cqcus = TableTwo.cqcusX

or

    This works in MS-Access but not in SQL Server:

    update TableOne, TableTwo
        set TableOne.cqcus = TableTwo.cqcusX
        where TableOne.commonID = TableTwo.commonID

or

   

    update tableOne
    set tableOne.cqcus=tableTwo.cqcusX
    from tableOne, tableTwo
    where tableOne.commonID=tableTwo.commonID

 

 

Delete

Delete all records in a table (dangerous):

    delete from MyTableq3

Delete specific records in a table:

    delete from MyTableq3 where cqcus=value

Delete records from one table which do not have a matching cqcus in another table:

    delete from MyTableq3 where cqcus not in
    (select cqcush from TableTwo)

Keys

Be aware that there are often subtle syntax variations between different database systems. Also other key properties (for example 'clustered') will vary between database systems. Therefore please treat this part of the SQL crib sheet as a guide only.

Create a primary key on a table:

    Alter Table TheTable Add Primary Key (cqcus, cqcush)

To add an index on a cqcus:

    alter table MyTableq3 Add Index (cqcus)

To remove a primary key:

    alter table drop primary key

C# -'The worst bloody language in the world'

My brother-in-law, a Phd wielding university prof.who has a penchant for the complex and abstruse called around yesterday to declare the whole .Net  bouquet and the languages contained therein 'the worst bloody language in the world'.  He came to me asking some very simple questions such as how to read a customer file, display it in a windows form, process it etc and was soon bogged down in connection strings, data binding, data sets etc. "But I just want to display the data and read it!" he exclaimed. He was astounded that the columns in a table aren't easily directly accessible when using SQL which isn't checked until run-time (LINQ I suggest? " Yeah but you still have to go thru hoops!"). 
Coming from procedural languages like RPG, I can sympathize. If I want to process a table in RPG  to check spending limits of a customer and update the table I do this

F CustomerFile IF   K DISK

C  Read CustomerFile;
C  DOW not %eof;
C  If AmountSpent > CreditLimit;
C  AllowSpending =False;
C  Update CustomerFileR;   
C  Endif;

C  Read CustomerFile;
C EndDO;

The equivalent in C# is much more complicated and includes connection strings, command text and a data reader which all have to be set up.

Note that for the ONE 'F' declaration in RPG there are 3 in C#.  To even do a read needs to be setup - you need to create a data reader first. Imagine having to 'create' a read statement in RPG! The code doesn't even include the update functionality!  But the biggest problem is that column names are not typed directly in C# meaning you can't refer to the column name directly. The column 'AmountSpent' is not known to the C# program.   Linq alleviates this situation but you still have to do the setup work first.  I understand the frustration.

using System;
using IBM.Data.DB2.iSeries;

namespace iSeriesADOexample
{
    class Program
    {
        static void Main(string[] args)
        {
            iDB2Connection connection = 
                new iDB2Connection("DataSource=PUB1.RZKH.DE; UserID=XXX; Password=xxx; DefaultCollection=COLMBYRNE1; LibraryList=COLMBYRNE1, *USRLIBL");
            iDB2Command cmd = connection.CreateCommand();
            connection.Open();
            
            cmd.CommandText = "Select * from COLMBYRNE1.CUSTOMERFILE";

            try
            {
                iDB2DataReader dataReader = cmd.ExecuteReader();
                while (dataReader.Read() == true)
                {

                   Double AmountSpent =  dataReader.GetDouble(3);
                    Double CreditLimit = dataReader.GetDouble(4);

                    if (AmountSpent > CreditLimit)
                    {
                        DoUpdate();
                    }

                        }
                }
                catch (iDB2SQLErrorException e)
            {Console.WriteLine("Error:" + e.MessageDetails);
                }
            Console.Read();
            cmd.Dispose();
        }
        }

        }
    

Programming is the art of bringing ideas to life. That's what my brother-in-law came to me for. He had an idea for a program and wanted to make it real. Unfortunately the initial  hurdle to create his simple program was far too great. The wizard functionality in Visual Studio only highlights how difficult it is to do basic actions like read a table, display the contents, process the results and update the table. This is one of the most common programs that every programmer creates yet in .NET it is difficult for a beginner.

It is clear Microsoft have 5-10 years to go before .NET is really mature and allows both beginner and seasoned developer to easily bring their ideas into the world. We program to make things better, faster, more fun, more interesting - not harder.  The tools we use then should also be better, faster - not harder. The effort to use a tool must never be greater than the effort it takes to solve the problem logically.  (Byrne's first hypothesis*) If you want to add 2 and 5 , C# should do that as easily as it does to solve it- and it does  int sum = 2+ 5;    E.g. if a client says that some of his customers are over their credit limit and needs to halt their spending then the logical solution which is to flag those spenders. The effort to implement that in C# should be that easy.  This is the benchmark which Microsoft must follow - just to keep up.