Improve documentation around :dbtype

Expose `next.jdbc.connection/dbtypes` which is a table of all known 
`:dbtype` values and their driver class names, ports, etc.

CHANGELOG.md

@@ -7,6 +7,7 @@ Only accretive/fixative changes will be made from now on.
 The following changes have been committed to the **master** branch since the 1.0.1 release:
 
 * Fix #40 by adding `next.jdbc.prepare/execute-batch!`.
+* Expose `next.jdbc.connect/dbtypes` as a table of known database types and aliases, along with their class name(s), port, and other JDBC string components.
 * Improved docstrings and documentation, especially around prepared statement handling.
 
 ## Stable Builds

doc/getting-started.md

@@ -61,7 +61,7 @@ user=>
 ```
 We described the database with just `:dbtype` and `:dbname` because it is created as a local file and needs no authentication. For most databases, you would need `:user` and `:password` for authentication, and if the database is running on a remote machine you would need `:host` and possibly `:port` (`next.jdbc` tries to guess the correct port based on the `:dbtype`).
 
-> Note: You can see the full list of `:dbtype` values supported in [next.jdbc/get-datasource](https://cljdoc.org/d/seancorfield/next.jdbc/CURRENT/api/next.jdbc#get-datasource).
+> Note: You can see the full list of `:dbtype` values supported in [next.jdbc/get-datasource](https://cljdoc.org/d/seancorfield/next.jdbc/CURRENT/api/next.jdbc#get-datasource)'s docstring. If you need this programmatically, you can get it from the [next.jdbc.connection/dbtypes](https://cljdoc.org/d/seancorfield/next.jdbc/CURRENT/api/next.jdbc.connection#dbtypes) hash map. If those lists differ, the hash map is the definitive list (and I'll need to fix the docstring!). The docstring of that Var explains how to tell `next.jdbc` about additional databases.
 
 We used `execute!` to create the `address` table, to insert a new row into it, and to query it. In all three cases, `execute!` returns a vector of hash maps with namespace-qualified keys, representing the result set from the operation, if available. When no result set is produced, `next.jdbc` returns a "result set" containing the "update count" from the operation (which is usually the number of rows affected). By default, H2 uses uppercase names and `next.jdbc` returns these as-is.
 

src/next/jdbc/connection.clj

@@ -63,6 +63,45 @@
    "sqlserver"  ";DATABASENAME="
    "oracle:sid" ":"})
 
+(def dbtypes
+  "A map of all known database types (including aliases) to the class name(s)
+  and port that `next.jdbc` supports out of the box. Just for completeness,
+  this also includes the prefixes used in the JDBC string for the `:host`
+  and `:dbname` (which are `//` and `/` respectively for nearly all types).
+
+  For known database types, you can use `:dbtype` (and omit `:classname`).
+
+  If you want to use a database that is not in this list, you can specify
+  a new `:dbtype` along with the class name of the JDBC driver in `:classname`.
+  You will also need to specify `:port`. For example:
+
+     `{:dbtype \"acme\" :classname \"com.acme.JdbcDriver\" ...}`
+
+  JDBC drivers are not provided by `next.jdbc` -- you need to specify the
+  driver(s) you need as additional dependencies in your project. For
+  example:
+
+     `[com.acme/jdbc \"1.2.3\"] ; lein/boot`
+  or:
+     `{com.acme/jdbc {:mvn/version \"1.2.3\"}} ; CLI/deps.edn`
+
+  Note: the `:classname` value can be a string or a vector of strings. If
+  a vector of strings is provided, an attempt will be made to load each
+  named class in order, until one succeeds. This allows for a given `:dbtype`
+  to be used with different versions of a JDBC driver, if the class name
+  has changed over time (such as with MySQL)."
+  (let [dbs (merge (zipmap (keys classnames) (keys classnames)) aliases)]
+    (reduce-kv (fn [m k v]
+                 (assoc m
+                        k
+                        (cond-> {:classname        (classnames v)
+                                 :host-prefix      (host-prefixes v "//")
+                                 :dbname-separator (dbname-separators v "/")}
+                          (ports v)
+                          (assoc :port (ports v)))))
+               {}
+               dbs)))
+
 (defn- ^Properties as-properties
   "Convert any seq of pairs to a `java.util.Properties` instance."
   [m]