SQL won't allow you to create dynamic and changing in-game effects. As
@TopHatPaladin said, lua is required for this.
SQL is where XML code actually goes -- they both add or alter things in the game's database. XML is translated into SQL and then inserted into the game's database. Once the game is started, the database is locked. So if the database says a building gives +1 Faith, nothing can be done by database methods to make it give dynamicaly more than or less than that +1 Faith depending on in-game conditions like a golden age being active. We have to turn to lua for that sort of effect.
Directly effecting a plot's yields via lua is possible but has many issues associated with it. If your lua code does not track whether or not that plot has already been adjusted, you get compound effects on the same tile, for example. And then you need to use a data-saving method to store the information about which tiles have been adjusted so that when a user saves and then later reloads that saved game, the lua code does not compound an effect on a tile that has already been adjusted.
One method mod-maker's often use to get around these these data-tracking and data-saving issues is to use dummy buildings, each of which for example gives +1 Faith. Then we just count via lua the number of tiles that qualify for the + Faith effect, and add that number of dummy buildings to a city. The game allows us to add more than one copy of the same building to a city in Civ5, and lua can later on access and count how many of a given building have been added to a city. Lua can then also take away all these dummy buildings as conditions require.
