Avoid $c(0) in globals

Are you sure that, your insert queries send null and not an empty string? Null and empty strings are different, what are you trying to achieve it's a null, and $c(0) is an empty string.

Documentation

$c(0) is the internal representation of the SQL empty string ''.  Your insert must be using '' as the value for myProperty instead of null if you are seeing $c(0) for the field values.  SQL null and empty-string string are very different.  null means unknown, while empty string is a 0 length string value.

If you always want to convert '' to null for myProperty values, write a myPropertyNormalize property member method that converts the value to null.  This will override the %String Normalize() method and convert $c(0) to "" whenever the property/field's value is normalized by the filers.

Trying to do this in a trigger will not work as triggers are not meant to be used to modify property/field values being filed.

Hi

if you have a simple Insert SQL statement in the form:

INSERT into {Table} VALUES ('abc','zyx',123,'',NULL)

then Cache SQL will replace '' with $c(0) in the global record and "" for NULL.

NULL is a valid keyword in SQL not only for testing conditions such as "WHERE {column} IS NOT NULL" but also when being used in the context of values in my INSERT example and also in Class Methods or Class Queries that are projected as SQL Stored Procedures. 

So if you have a Class Method projected as a Stored Procedure then if you call the stored procedure as follows:

call MyTable_MyProcedure('abc','xyz',123,'',NULL) 

then the values for the parameters will be

p1 = "abc"

p2 = "xyz"

p3 = 123

p4 = $c(0)

p5 = ""

Yours

Nigel

There is another way.
If you want any empty strings to always be treated as null/"" instead of "/$c(0), then there is an documented setting (within the scope of namespace), namely:

^%SYS("sql","sys","namespace",<YOUR_NAMESPACE>,"empty string")

Here is a small example:

Class dc.test Extends %Persistent
{

Property i As %Integer;

Property str As %String;

/// do ##class(dc.test).Test()
ClassMethod Test()
{
 do ..%KillExtent()
 
 try{
  
  do $system.SQL.Purge()
  set ^%SYS("sql","sys","namespace",$namespace,"empty string")=$c(0)

  ; '' -> $c(0)
  do ##class(%SQL.Statement).%ExecDirect(,"insert into dc.test(i,str) values(1,'')")
  
  ; null -> ""
  do ##class(%SQL.Statement).%ExecDirect(,"insert into dc.test(i,str) values(2,null)")
  
  do $system.SQL.Purge()
  set ^%SYS("sql","sys","namespace",$namespace,"empty string")=""

  ; '' -> ""
  do ##class(%SQL.Statement).%ExecDirect(,"insert into dc.test(i,str) values(11,'')")
  
  ; null -> ""
  do ##class(%SQL.Statement).%ExecDirect(,"insert into dc.test(i,str) values(22,null)")

  zwrite ^dc.testD
  kill ^%SYS("sql","sys","namespace",$namespace,"empty string")
 }
 catch(ex){
  write ex.DisplayString(),!
 }
}

}

USER>do ##class(dc.test).Test()
^dc.testD=4
^dc.testD(1)=$lb("",1,$c(0))
^dc.testD(2)=$lb("",2,"")
^dc.testD(3)=$lb("",11,"")
^dc.testD(4)=$lb("",22,"")