Add SQL replacer notes

sql/README.md

@@ -0,0 +1,53 @@
+# SQL string macros
+To make writing SQL queries for gochan that can be used on MySQL, Postgresql, and SQLite easier without having to write a bunch of `switch sqlConfig.DBType` blocks or `query = "SELECT * FROM " + sqlConfig.DBprefix + "table..."`, gochan uses a replacer that replaces certain strings with an appropriate string when running queries through the `gcsql` package.
+
+## Configuration-based replacers
+Input    | Output
+---------|--------
+DBPREFIX | value of `config.SQLConfig.DBprefix`
+DBNAME   | value of `config.SQLConfig.DBname`
+
+## SQL implementation-based replacers
+
+Input                        | MySQL/MariaDB           | Postgresql  | SQLite
+-----------------------------|-------------------------|-------------|-------------------
+?<sup>1</sup>                | ?                       | $#          | $#
+RANGE_START_ATON<sup>2</sup> | INET6_ATON(range_start) | range_start | range_start
+RANGE_START_NTOA             | INET6_NTOA(range_start) | range_start | range_start
+RANGE_END_ATON               | INET6_ATON(range_end)   | range_end   | range_end
+RANGE_END_NTOA               | INET6_NTOA(range_end)   | range_end   | range_end
+IP_ATON                      | INET6_ATON(ip)          | ip          | ip
+IP_NTOA                      | INET6_NTOA(ip)          | ip          | ip
+PARAM_ATON                   | INET6_ATON(?)           | $#          | $#
+PARAM_NTOA                   | INET6_NTOA(?)           | $#          | $#
+
+## Example
+```Go
+// SQL implementation-independent usage style used by gochan
+rows, err := gcsql.QuerySQL("SELECT id, reason, RANGE_START_NTOA, RANGE_END_NTOA FROM DBNAME.DBPREFIXip_bans WHERE RANGE_START_ATON = PARAM_ATON", "192.168.56.1")
+```
+The above code essentially does the same as the bottom code
+```Go
+sqlConfig := config.GetSQLConfig()
+dbPrefix := sqlConfig.DBprefix
+dbName := sqlConfig.DBname // probably not necessary for most queries, but included here for documentation
+db := gcsql.GetBaseDB() // don't use this unless you know what you're doing and really need to access the *sql.DB object
+
+var stmt *sql.Stmt
+var err error
+switch sqlConfig.DBtype {
+case "mysql":
+    stmt, err = db.Prepare("SELECT id, reason, INET6_NTOA(range_start), INET6_NTOA(range_end) FROM " + dbName + "." + dbPrefix + "ip_bans WHERE INET6_ATON(range_start) = INET6_ATON(?)")
+case "postgres":
+    stmt, err = db.Prepare("SELECT id, reason, range_start, range_end FROM " + dbName + "." + dbPrefix + "ip_bans WHERE range_start = $1")
+case "sqlite3":
+    stmt, err = db.Prepare("SELECT id, reason, range_start, range_end FROM " + dbName + "." + dbPrefix + "ip_bans WHERE range_start = $1")
+}
+// error handling here
+defer stmt.Close()
+rows, err := stmt.Query("192.168.56.1")
+```
+
+## Notes
+1. For numbered query parameters (which are exclusively used in gochan as of this writing, as opposed to named parameters, which may or may not end up replacing them), MySQL and MariaDB use `?` for every parameter. Postgresql and SQLite use $ followed by the parameter number (e.g., `$1`, `$2`, ...)
+2. Although SQLite can store IP addresses in a `VARBINARY` column like MySQL, SQLite does not have a built-in function for converting them to and from a string like MySQL's `INET6_NTOA()` and `INET6_ATON()`, or a built-in data type to compare them to string parameters like Postgres, so `range_start < PARAM_ATON` will not work when using the sqlite3 driver.