How can I check if OpenSQLConnection was successful (in Mathematica)? - wolfram-mathematica

Question

How can I check if DatabaseLink`OpenSQLConnection was successful? My code is as follows
conn = OpenSQLConnection[JDBC["hsqldb", "file"], "Name"-> "test"];
Can I use something like Head[conn]?

Answer 1

The return value for successful connection will have head SQLConnection (in the DatabaseLink context)
More generally:
OpenSQLConnection returns $Failed when the connection failed for whatever reason:
In[25]:= OpenSQLConnection[JDBC["mysql", "localhost:3306/foo"],
"Username" -> "foo", "Password" -> "bar"]
During evaluation of In[25]:= JDBC::error: Access denied for user 'foo'#'localhost' (using password: YES) >>
Out[25]= $Failed
... and unevaluated when its arguments were not of the proper form:
In[28]:= OpenSQLConnection[Sin[x]]
Out[28]= OpenSQLConnection[Sin[x]]
Therefore, you can look for a return value of $Failed and optionally also use Check[...] to trap and handle messages that were generated. As you guessed, you can use Head[returnvalue] to make sure the head of the return value does not equal OpenSQLConnection.

Answer 2

This is not exactly an answer to your question, but this is what I do to connect reliably to my database:
Needs["DatabaseLink`"];
CloseSQLConnection[conn];
TimeConstrained[
conn=OpenSQLConnection[ JDBC["mysql","localhost:3306/mydb"],
"Username"->"myuser",
"Password"->"mypw"],
5,
CloseSQLConnection[conn];
conn=OpenSQLConnection[ JDBC["mysql","localhost:3306/mydb"],
"Username"->"myuser",
"Password"->"mypw"]
];