Document type-conversions in Scan(); fix #38

Author: xaprb

retrieving.md

@@ -79,6 +79,27 @@ Second, you should always check for an error at the end of the `for rows.Next()`
 loop. If there's an error during the loop, you need to know about it. Don't just
 assume that the loop iterates until you've processed all the rows.
 
+How Scan() Works
+================
+
+When you iterate over rows and scan them into destination variables, Go performs data
+type conversions work for you, behind the scenes. It is based on the type of the
+destination variable. Being aware of this can clean up your code and help avoid
+repetitive work.
+
+For example, suppose you select some rows from a table that is defined with
+string columns, such as `VARCHAR(45)` or similar. You happen to know, however,
+that the table always contains numbers. If you pass a pointer to a string, Go
+will copy the bytes into the string. Now you can use `strconv.ParseInt()` or
+similar to convert the value to a number. You'll have to check for errors in the
+SQL operations, as well as errors parsing the integer. This is messy and
+tedious.
+
+Or, you can just pass `Scan()` a pointer to an integer. Go will detect that and
+call `strconv.ParseInt()` for you. If there's an error in conversion, the call
+to `Scan()` will return it. Your code is neater and smaller now. This is the
+recommended way to use `database/sql`.
+
 Preparing Queries
 =================