Dynamically building WHERE clauses in SQL statements - sql

Question

I have a question regarding SQL. I have the following SQL statement:
SELECT id, First, Last, E_Mail, Notes
FROM mytable
WHERE SOMETHING_SHOULD_BE_HERE IS NOT NULL;
I know that the SOMETHING_SHOULD_BE_HERE should be a column(attribute) in my table. Is their a way I can put a variable that can refer to the column I'm trying to access? In my case their are 30 columns. Can I have a string for SOMETHING_SHOULD_BE_HERE that can be assigned in my program to the column in which I want to search?
Thanks

Answer 1

No. Variables in SQL can refer to data, but not to object names (columns, functions or other database objects).
If you are building the SQL query, you'll need to use string operations to build your query.

Answer 2

The column can't be variable, but the value of the column can. The parser needs to know what to bind to.
If you elaborate on what you're trying to solve and which platform you're using it would allow for more complete answers.

Answer 3

You can have different SQLs queries in your code and use each one according to the case.
Another way is generate dynamically the query according the fields you want.

Answer 4

Without dynamic SQL, this is probably your best bet:
SELECT
id, first, last, email, notes
FROM
My_Table
WHERE
CASE #column_name_variable
WHEN 'column_1' THEN column_1
WHEN 'column_2' THEN column_2
...
ELSE 'not null'
END IS NOT NULL
There might be some issues with data type conversions, so you might need to explicitly cast all of the columns to one data type (VARCHAR is probably the best bet). Also, there's a good chance that performance will be horrendous on this query. I'd test it thoroughly before even thinking about implementing something like this.
I mentioned this in my comment, but for completeness I'll put it here too... you can probably also accomplish this with dynamic SQL, but how you do that will depend on your database server (MS SQL Server, Oracle, mySQL, etc.) and there are usually some caveats to using dynamic SQL.

Answer 5

In JDBC program, yes,the select statement can be composed like string operation.
for(String colName: colList)
{
String sql="Select id, First, Last, E_Mail, Notes From mytable where "+colName+" IS NOT NULL";
//execute the sql statement
}

Answer 6

It depends on how you are going to find out the value of SOMETHING_SHOULD_BE_HERE.
If you are in an Oracle PLS/SQL environment you could build up the WHERE clause using dynamic SQL and then use EXECUTE IMMEDIATE to execute it.
If you have a small set number of possibilities you could use CASE to workaround your problem possibly.

Answer 7

Your question is unclear.
However I am quite sure that what you have in mind is the so-called dynamic SQL (and related). "Dynamic SQL" allows you to dynamically build and submit queries at runtime. However such functionalities may not exist for your RDBMS.
There are several ways to do this.
When your query would return one and only one row
then you have to consider the EXECUTE IMMEDIATE statements (along with sp_executesql in tSQL : http://msdn.microsoft.com/en-us/library/ms188001.aspx ; or the USING clause in PL/SQL : http://docs.oracle.com/cd/B14117_01/appdev.101/b10807/13_elems017.htm to specify a list of input/output bind arguments) and/or PREPARED statements (http://rpbouman.blogspot.fr/2005/11/mysql-5-prepared-statement-syntax-and.html).
When your query can return more than one row
then you have to consider techniques such as the EXECUTE IMMEDIATE statement with the BULK COLLECT INTO clause or the OPEN-FOR, FETCH, and CLOSE statements (explicit cursors in PL/SQL :
http://docs.oracle.com/cd/B28359_01/appdev.111/b28370/dynamic.htm)
Please note that except in some particular cases, most conventional techniques like IF-THEN-ELSE and CASE statements should be preferred (along with a good algorithm). Furthermore they work with almost all RDBMS.