Order-based Calculations – Compare First Two Records in a Group

Question
I have given data as strings. There are two tables, one with the main data and the other with historical values.

Tablestocks:

+———-+——-+———–+

| stock_id | symbol| name |

+———-+——-+———–+

| 1 | AAPL | Apple |

| 2 | GOOG | Google |

| 3 | MSFT | Microsoft |

+———-+——-+———–+

Tableprices:

+———-+——-+———————+

| stock_id | price | date |

+———-+——-+———————+

| 1 | 0.05 | 2015-02-24 01:00:00 |

| 2 | 2.20 | 2015-02-24 01:00:00 |

| 1 | 0.50 | 2015-02-23 23:00:00 |

| 2 | 1.90 | 2015-02-23 23:00:00 |

| 3 | 2.10 | 2015-02-23 23:00:00 |

| 1 | 1.00 | 2015-02-23 19:00:00 |

| 2 | 1.00 | 2015-02-23 19:00:00 |

+———-+——-+———————+

I need a query that returns:

+———-+——-+———–+——-+

| stock_id | symbol| name | diff |

+———-+——-+———–+——-+

| 1 | AAPL | Apple | -0.45 |

| 2 | GOOG | Google | 0.30 |

| 3 | MSFT | Microsoft | NULL |

+———-+——-+———–+——-+

diff is the result of subtracting from the newest price of a stock the previous price. If one or less prices are present for a particular stock I should get NULL.

I have the following queries that return the last price and the previous price but I don’t know how to join everything

/* last */

SELECT price

FROM prices

WHERE stock_id = ‘1’

ORDER BY date DESC

LIMIT 1

/* previous */

SELECT price

FROM prices

WHERE stock_id = ‘1’

ORDER BY date DESC

LIMIT 1,1