LEFT OUTER JOIN with subquery syntax

Lets build this up slowly.

First, lets see about getting just the information about stars:

SELECT name AS starName, (class + 7) * intensity * 1000000 AS starTemp FROM StarsWHERE starId < 100

(this should look might familiar!)
We get a list of all stars whose starId is less than 100 (the WHERE clause), grabbing the name and calculating temperature. At this point, we don't need a disambiguating reference to source.

Next, we need to add planet information. What about an INNER JOIN (note that the actual keyword INNER is optional)?

SELECT Stars.name as starName, (Stars.class + 7) * Stars.intensity * 1000000 AS starTemp,       Planets.name as planetNameFROM StarsINNER JOIN Planets        ON Planets.starId = Stars.starIdWHERE Stars.starId < 100

The ON clause is using an = (equals) condition to link planets to the star they orbit; otherwise, we'd be saying they were orbiting more than one star, which is very unusual! Each star is listed once for every planet it has, but that's expected.

...Except now we have a problem: Some of our stars from the first query disappeared! The (INNER) JOIN is causing only stars with at least one planet to be reported. But we still need to report stars without any planets! So what about a LEFT (OUTER) JOIN?

SELECT Stars.name as starName, (Stars.class + 7) * Stars.intensity * 1000000 AS starTemp,       Planets.name as planetNameFROM StarsLEFT JOIN Planets       ON Planets.starId = Stars.starIdWHERE Stars.starId < 100

... And we have all the stars back, with planetName being null (and only appearing once) if there are no planets for that star. Good so far!

Now we need to add the planet temperature. Should be simple:

SELECT Stars.name as starName, (Stars.class + 7) * Stars.intensity * 1000000 AS starTemp,       Planets.name as planetName, starTemp - (50 * Planets.orbitDistance) as planetTempFROM StarsLEFT JOIN Planets       ON Planets.starId = Stars.starIdWHERE Stars.starId < 100

...except that on most RDBMSs, you'll get a syntax error stating the system can't find starTemp. What's going on? The problem is that the new column alias (name) isn't (usually) available until after the SELECT part of the statement runs. Which means we need to put in the calculation again:

SELECT Stars.name as starName, (Stars.class + 7) * Stars.intensity * 1000000 AS starTemp,       Planets.name as planetName,        ((Stars.class + 7) * Stars.intensity * 1000000) - (50 * Planets.orbitDistance) as planetTempFROM StarsLEFT JOIN Planets       ON Planets.starId = Stars.starIdWHERE Stars.starId < 100

(note that the db may actually be smart enough to perform the starTemp calculation only once per-line, but when writing you have to mention it twice in this context).
Well, that's slightly messy, but it works. Hopefully, you'll remember to change both references if that's necessary...

Thankfully, we can move the Stars portion of this into a subquery. We'll only have to list the calculation for starTemp once!

SELECT Stars.starName, Stars.starTemp,       Planets.name as planetName,        Stars.starTemp - (50 * Planets.orbitDistance) as planetTempFROM (SELECT starId, name AS starName, (class + 7) * intensity * 1000000 AS starTemp       FROM Stars      WHERE starId < 100) StarsLEFT JOIN Planets       ON Planets.starId = Stars.starId

Yeah, that looks like how I'd write it. Should work on essentially any RDBMS.

Note that the parenthesis in Stars.starTemp - (50 * Planets.orbitDistance) is only there for clarity for the reader, the meaning of the math would remain unchanged if they were removed. Regardless of how well you know operator-precedence rules, always put in parenthesis when mixing operations. This becomes especially beneficial when dealing with ORs and ANDs in JOIN and WHERE conditions - many people lose track of what's going to be effected.
Also note that the implicit-join syntax (the comma-separated FROM clause) is considered bad practice in general, or outright deprecated on some platforms (queries will still run, but the db may scold you). It also makes certain things - like LEFT JOINs - difficult to do, and increases the possibility of accidently sabotaging yourself. So please, avoid it.

SELECT * FROM (SELECT [...]) as Alias1LEFT OUTER JOIN     (SELECT [...]) as Alias2ON Alias1.id = Alias2.id

WITH(  SELECT    stars.name AS starname, ((star.class+7)*star.intensity)*1000000) AS startemp,   stars.starid FROM   stars) AS star_tempsSELECT    planets.name AS planetname, (startemp-50*planets.orbitdistance) AS planettemp   star_temps.starname, star_temps.startempFROM    star_temps LEFT OUTER JOIN planets USING (star_id)WHERE   star_temps.starid < 100;

Alternatively, one can construct a sub-query (I've used a common table expression) to accomplish the same task as demonstrated below:

SELECT    planets.name AS planetname, (startemp-50*planets.orbitdistance) AS planettemp   star_temps.starname, star_temps.startempFROM    (SELECT       stars.name AS starname, ((star.class+7)*star.intensity)*1000000) AS startemp,      stars.starid    FROM      stars ) AS star_tempsLEFT OUTER JOIN planets USING (star_id)WHERE   star_temps.starid < 100;