SQL WHERE - the easy way

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Creating SQL WHERE clauses

When developing C++ code to query SQL databases, one ends up with code which contains lots of ugly and cluttering statements like this:

	
char buf[256]; 
sprintf(buf, "WHERE (article=%i) AND (subject='%s%')", nArticle, pstrSubject); 
strcat(sql, buf);
// or even worse
strcat(sql, " WHERE (article=");
strcat(sql, _itoa(nArticle, buf, 10));
strcat(sql, ") AND (subject='");
strcat(sql, pstrSubject);
strcat(sql, ")");
// and so on and so forth...

Most of you have seen that sort of thing. I created the CWhere class to simplify creation of WHERE clauses and cleanup code. The resulting code, using the CWhere class, looks like this:

	
CWhere w(_T("article"), 1234); 
w.Add(_T("subject"), _T("CodeProject"); 
mySQLstring += w.GetWhereClause(); 
// should read "WHERE (article = 1234) AND (subject = 'CodeProject') ";

The class implements various methods to create the final string. Basically it all ends in providing several conversions of various datatypes to strings.
Currently the class contains all functions needed to create the following kind of clauses:

1. "fieldXY IS NULL"
2. "fieldXY exp value" where exp is any valid SQL operator (<, >, =,...)
3. "fieldXY BETWEEN value1 AND value2"

Also, these clauses can be cascaded with AND and OR statements. The class takes care of nice SQL formatting, paranthesis, brackets and spaces. You can immediately use the resulting string for concatenation or other string operations. It contains leading and trailing spaces and all expressions are enclosed in brackets.

The class will provide correct strings for Unicode SQL statements too when the application is compiled using the UNICODE define.

Using the class

Lets start with the most common usage: creating a instance of CWhere with the first expression supplied in the constructor:

CWhere w(_T("article"), 1234);

This will create the variable w already containing "WHERE (article = 1234)". Now lets add another field:

w.Add(_T("subject"), _T("CodeProject");

This will add a " AND (subject = 'CodeProject')" to the variable. Finally, use the WHERE clause and add it to an existing SQL statement created somewhere else:

mySQLstring += w.GetWhereClause(); 
// should contain " WHERE (article = 1234) AND (subject = 'CodeProject') "

If the presence of the keyword WHERE is not needed or not wanted, just use GetWhereClausePlain().

Logical operations AND, OR

To simplify the creation of complex clauses containing AND or OR combinations, the operators &, |, &= and |= are overloaded. You can use them in a very intuitive way to create logical operations with the CWhere class.

	
CWhere w, article1, article2("article", -1L), subject("subject", pstrSubject), priority("priority", 2);
// logical operation OR
w = article1 | article2;		
// logical operation AND
w &= subject & priority;		

The resulting WHERE clause is: " WHERE ((article IS NULL) OR (article = -1)) AND ((subject = 'CodeProject') AND (priority = 2))"

Compatibility

Tested under Windows XP Pro, with VC6 and MFC. Supports fully UNICODE or MBCS. STL version is up to the reader