将字符串转换为数字DB2实体框架

Question

I'm migrating a legacy DB2 Application to a web application and I'm using ASP.NET WebAPI with entity framework.

This is the first time I'm working with DB2.

I have installed the necessary drivers and fix packs for DB2 database.I have generated the EDMX for existing DB2 database tables.

Everything works fine. I have encountered a strange issue that, In one of the DB2 SQL query, a string column is compared with a number value.

Consider the below query

SELECT DISTINCT DB23.EMPLOYEES.FIRST_NAME,   
    DB23.EMPLOYEES.LAST_NAME,   
    STRIP("DB23"."EMPLOYEES"."STATE_NAME") STATE_NAME, 
    DB23.CONTACT.PHONE_NO
FROM DB23.EMPLOYEES,   
     DB23.CONTACT 
WHERE ( DB23.EMPLOYEES.EMP_ID = DB23.CONTACT.EMP_ID ) and     
     ( DB23.CONTACT.EMP_REG_NO < '900000' )   
ORDER BY DB23.EMPLOYEES.FIRST_NAME ASC

here the column EMP_REG_NO is a string type and it is compared with a number using less than ' < ' operator and the strange thing is that the same query runs perfectly in IBM DB2 Server Client.

The below one is the Linq expression that I have written

from emp in _DB2Context.EMPLOYEES
join cont in _DB2Context.CONTACT on emp.EMP_ID equals cont.EMP_ID
where cont.EMP_REG_ID < 900000
select new {
   emp.FIRST_NAME
   emp.LAST_NAME
   emp.STATE_NAME
   cont.PHONE_NO
};

Here we cannot compare the number 900000 with string type column.

I have tried like this

from emp in _DB2Context.EMPLOYEES
join cont in _DB2Context.CONTACT on emp.EMP_ID equals cont.EMP_ID
where Convert.ToInt64(cont.EMP_REG_ID) < 900000
select new {
   emp.FIRST_NAME
   emp.LAST_NAME
   emp.STATE_NAME
   cont.PHONE_NO
};

This gave me an error that LINQ doesn't recognize method Convert.ToInt64() and I understood that There is no equivalent method to achieve the conversion in T-SQL statement.

Any Ideas?

Update:

The below is the table structure

TABLE - EMPLOYEES
***************************************************************************

Name            Data Type     Type schema   Length   Scale   Nulls Hidden
--------------- ------------- ------------- -------- ------- ----- --------
EMP_ID          CHARACTER     SYSIBM               2       0 N     Not 
FIRST_NAME      CHARACTER     SYSIBM              30       0 N     Not     
LAST_NAME       CHARACTER     SYSIBM              30       0 N     Not     
STATE_NAME      CHARACTER     SYSIBM              25       0 N     Not     
FIPS_CTRY_CD    CHARACTER     SYSIBM               2       0 N     Not    
POLK_STATE_CD   CHARACTER     SYSIBM               2       0 N     Not     
MAIL_STATE      CHARACTER     SYSIBM              15       0 N     Not    

****************************************************************************

TABLE - CONTACT
****************************************************************************

Name            Data Type     Type schema   Length   Scale   Nulls Hidden
--------------- ------------- ------------- -------- ------- ----- --------
EMP_ID          CHARACTER     SYSIBM               2       0 N     Not 
REG_NO          CHARACTER     SYSIBM               6       0 N     Not     
REG_STATE_CD    CHARACTER     SYSIBM               2       0 Y     Not     
PHONE_NO        CHARACTER     SYSIBM              15       0 N     Not     
REG_STATE       CHARACTER     SYSIBM               2       0 N     Not     
CORP_NAME       CHARACTER     SYSIBM              30       0 N     Not     

***************************************************************************

The below is the POCO classes generated by the entity framework

public partial class EMPLOYEES
{
    public string EMP_ID { get; set; }
    public string FIRST_NAME { get; set; }
    public string LAST_NAME { get; set; }
    public string STATE_NAME { get; set; }
    public string FIPS_CTRY_CD { get; set; }
    public string POLK_STATE_CD { get; set; }
    public string MAIL_STATE { get; set; }
}

public partial class CONTACT
{
    public string EMP_ID { get; set; }
    public string REG_NO { get; set; }
    public string REG_STATE_CD { get; set; }
    public string PHONE_NO { get; set; }
    public string REG_STATE { get; set; }
    public string CORP_NAME { get; set; }
}

moreover, I can able to run the below simple query in DB2 command line processor

SELECT
 DB23.CONTACT.EMP_ID
FROM DB23.CONTACT 
WHERE DB23.CONTACT.EMP_ID < '900000'

SELECT
 DB23.CONTACT.EMP_ID
FROM DB23.CONTACT 
WHERE DB23.CONTACT.EMP_ID < 900000

Both DB2 Query returns result

EMP_ID
------
123   
234 

But I cannot do the same with LINQ like,

from cont in _DB2Context.CONTACT
where cont.DLR_NO < 90000  // here getting an error says that Operator '<' cannot be applied to operands of type 'string' and 'int'
select new {
  cont.DLR_NO
}

from cont in _DB2Context.CONTACT
where cont.DLR_NO < '90000'  // here getting an error says that Operator '<' cannot be applied to operands of type 'string' and 'char'
select new {
  cont.DLR_NO
}

Thanks, Gowrish.

Answer 1

Assuming that it will be translated into its SQL equivalent, try using something like

where cont.DLR_NO.CompareTo("90000") < 0

As for string to integer conversion, you have no guarantee that a CHARACTER column in the database will only ever contain digits, which makes any program that insists on casting that column to an integer a risky bet. In those situations, it's safer to convert your numeric literal to a string first (from 90000 to "90000" ) so you're simply comparing two strings.