So, recently I have, on two occasions ended up discussing the pro's and con's of different power generation systems. I thought it might be helpful to capture some of the arguments here and have a place where follow-ups could be noted. Some of the balance of the argument depends on geography, some on natural resources and sustainability over the long-term. I might have made some mistakes, so I would appreciate any input.

Fossil Fuels

Fossil fuel thus not long-term sustainable, they are created from ancient organic materials which have been compressed and baked until they turn into a combustable solid, liquid or gas. It takes millions of years to produce these materials and they cannot be replaced in the lifetime of our civilisation. Continous supply of energy as long as it is needed and possible to reduce output to match demand.

1) Natural Gas Fired

Western European countries Gas fields are increasingly depleated. Cleaner burning than many other fossil fuels and relatively efficient conversion to electricity. Scales from domestic generator to power-station with good efficiency.

2) Coal Fired

Mining coal is either a difficult and dangerous operation under ground, or it can be strip mined which leaves significant scaring on the landscape. Burning coal is relatively dirty.

3) Oil Fired

Difficult and dangerous extraction as shown by the Gulf of Mexico. Quite dirty generation.

Atomic / Nuclear

Typically continous supply which is quite reliable to meet demand, but may also be wasteful if the energy is not needed off-peak.

1) Uranium Fast Breeder Reactor

Principles designed over 50 years ago for a different age, sponsored by government because the by-product is weapons grade radioactive isotopes. Easy to generate large ammounts of electricity. Expensive plant design, long-term safety implications and difficult end-of-life management for the facility. Financially difficult to justify because of the end-of-life implications but with subsidies possibly one of the most powerful continous supply generators.

2) Thorium Molten Salt Reactor

Thorium is much more efficient to extract than Uranium and relatively safe to handle. When embedded in molten halide salts then it can easily be deactivated in the case of difficulties. The isotopes it produces have a fairly safe half-life and are not very radioactive. Also because the radioactive material is contained in a liquid it cannot suffer from physical stressing like a solid fuel.

Environmental Power

1) Wind turbines

Subject to mechanical stresses, so requires difficult maintenance. However can be constructed from sustainable materials and can be recycled. Heavy bases need to be constructed with concrete but can be reused. Not dependable and predictable, cannot be adjusted to meet a growth in demand. Subject to the availability of heavy winds, with no wind there is no power generated and has to be shut down in excessive wind. Possible environmental impact to wildlife, particularly birds, and some visual/noise impact. Good energy transfer from the mechanical wind to electricity.

2) Photovoltaic

Produced from a silicon chemical substrate, environmental impact in production and risk of pollution. Poor efficiency compared to carbon impact of manufacturing and transport. Power output is subject to the availability of good levels of sun.

3) Solar-thermal-electric

By focusing the sun on a boiler or Sterling generator a clean and sustainable electricity is generated. Subject to sun availability and still difficult to transfer but with potentially less polution in manufacturing than alternatives.

4) Geo-thermal

Using the heat of the earth to produce steam and generate electricity. Dependable source of energy, subject to regional effectiveness where pockets of hot earth are available for use.

5) Tidal/wave energy

Use of the power of the sea to turn generators. This is a very powerful and clean form of energy, in areas like the British Isles a fairly consistent output can be given. Probable environmental impacts on fishing and wildlife. There is enough sea energy on the west coast of Ireland to power the entire British Isles demands for energy.

6) Hydroelectric

Requires a massive geo-engineering effort involving large ammounts of concrete which has a highly polluting production. However once constructed it can have a long lifespan of clean production.

Bio Fuels

Biofuels are sources which can be burnt to release their energy which was usually gathered through the growing of plant materials. The carbon released is almost as much as that which was consumed in the growth. However this is at the sacrifice of land which can be used for growing food, with world food shortages it is a shame to be burning crops for energy.

Ok, so I have a table on an MySQL server, it has the following columns:

Key, UserID, datetime and position

Position can be "start", "login" and "end".

I want to find out the difference between start, sign in and end for each login event.

Anyone have an idea of an SQL query I could use? I think it has something to do with a query within a query?

I postulate that I would first SELECT the start event and then for each "start" event it should look for the next first login event (after the time of the start but less than one hour after the start) then DATE_SUB() to find out the difference. It would also be good to filter out "next" events that happen after a future "start" event in case end didn't happen.

I would be very grateful if anyone could save me doing this in Excel or Perl.