## `local mysql = require'mysql_client'`

MySQL client protocol in Lua.
Stolen from OpenResty, modified to work with [sock] and added prepared statements.

## Example

```lua
local mysql = require'mysql_client'

assert(mysql.connect{
	host = '127.0.0.1',
	port = 3306,
	user = 'bar',
	password = 'baz',
	schema = 'foo',
	charset = 'utf8mb4',
	max_packet_size = 1024 * 1024,
})

assert(cn:query('drop table if exists cats'))

local res = assert(cn:query('create table cats '
			  .. '(id serial primary key, '
			  .. 'name varchar(5))'))

local res = assert(cn:query('insert into cats (name) '
	.. "values ('Bob'),(''),(null)"))

print(res.affected_rows, ' rows inserted into table cats ',
		'(last insert id: ', res.insert_id, ')')

require'pp'(assert(cn:query('select * from cats order by id asc', 10)))

assert(cn:close())
```

## API

### `mysql.connect(options) -> ok | nil,err,errcode,sqlstate`

Connect to a MySQL server.

The `options` argument is a Lua table holding the following keys:

  * `host`: server's IP address (required).
  * `port`: server's port (optional, defaults to 3306).
  * `user`: user name.
  * `password`: password (optional).
  * `schema`: the schema to set as current schema (optional).
  * `collation`: the collation used for the connection (`charset` is implied by this).
   * use `'server'` to get the server's collation and charset for the connection.
  * `charset`: the character set used for the connection.
   * if `collation` not set, the default collation for the charset is selected.
  * `max_packet_size`: the upper limit for the reply packets sent from the server (defaults to 16 MB).
  * `ssl`: if `true`, then uses SSL to connect to MySQL (defaults to `false`).
  If the server does not have SSL support (or just disabled), the error string
  "ssl disabled on server" is returned.
  * `ssl_verify`: if `true`, then verifies the validity of the server SSL
  certificate (default is `false`).
  * `to_lua = f(v, col) -> v` -- custom value converter (defaults to `mysql.to_lua`).

### `cn:close() -> 1 | nil,err`

Closes the current mysql connection and returns the status.

### `cn:send_query(query) -> bytes | nil,err`

Sends the query to the remote MySQL server without waiting for its replies.

Returns the bytes successfully sent out. Use `read_result()` to read the replies.

### `cn:read_result([options]) -> res,nil|'again',cols | nil,err,errcode,sqlstate`

Reads in the next result set returned from the server.

The `options` arg can contain:

  * `compact   = true` -- return an array of arrays instead of an array of `{column->value}` maps.
  * `to_array  = true` -- return an array of values for single-column results.
  * `null_value = val` -- value to use for `null` (defaults to `nil`).
  * `to_lua = f(v, col) -> v` -- custom value converter.

For queries that return a result set, it returns an array of rows.
For other queries it returns a Lua table with information such as
the autoincrement value if any and the affected rows.

If more results are following the current result, a second return value
`'again'` is returned. One should always check this value and call this
method again to retrieve more results. This usually happens when the original
query contains multiple statements (separated by semicolon in the same
query string) or calling a stored procedure.

In case of errors, this method returns at most 4 values: `nil`, `err`, `errcode`, and `sqlstate`.
The `err` return value contains a string describing the error, the `errcode`
return value holds the MySQL error code (a numerical value), and finally,
the `sqlstate` return value contains the standard SQL error code that consists
of 5 characters. Note that, the `errcode` and `sqlstate` might be `nil`
if MySQL does not return them.

NOTE: 64 bit integers and decimals are converted to Lua numbers by default.
That limits the useful integer range of number types to ±2^51 or 15 digits.

### `cn:query(query, [options]) -> res,nil,cols | nil,err,errcode,sqlstate`

This is a shortcut for combining the [send_query](#send_query) call
and the first [read_result](#read_result) call.

You should always check if the `err` return value  is `again` in case of
success because this method will only call [read_result](#read_result)
once for you.


### `cn:prepare(query, [opt]) -> stmt`

Prepare a statement. Options can contain:

  * `cursor`: 'read_only', 'update', 'scrollabe', 'none' (default: 'none').

### `stmt:exec(params...)`

Execute a statement. Use `cn:read_result()` to get the results.

### `stmt:free()`

Free statement.

### `cn.server_ver`

The MySQL server version string.

### `cn:quote(s) -> s`

Quote literal string to be used in queries. Quoting only works if current
collation is known (ses `collation` arg on `connect()`).

### Multiple result set support

For a SQL query that produces multiple result-sets, it is always your duty to
check the 'again' error message returned by the query, and keep pulling more
result sets by calling the `read_result()` until no 'again' error message
returned (or some other errors happen).

## Limitations

### Authentication

By default, of all authentication methods, only
[Old Password Authentication(mysql_old_password)](https://dev.mysql.com/doc/internals/en/old-password-authentication.html)
and [Secure Password Authentication(mysql_native_password)](https://dev.mysql.com/doc/internals/en/secure-password-authentication.html)
are suppored.

## TODO

* implement the data compression support in the protocol.