[SalesForce] Difference between != and <> comparison operators

If you're looking for the value of a single variable, then it would make sense that the operator would be "equal" to the value of the specified variable, it wouldn't be "IN" the variable as if your query has to sort among a group of records.

According to syntax, if your variable is a list or set, you'd expect the value to be "IN" the list or set, not "equal" to the value of the list or set. Often times it makes no difference which one you use for a list or set as Apex is smart enough to understand what you intended. It could also be a question of did you want all the values contained in the list or set, or only some of them that makes the difference on which to use that clarifies the intent, but generally if a list or set, using "IN" would be the proper syntax.

Where it's essential to use "IN" is when you use a map. A value of an operator can't be "equal" to a map can it? Instead, it has to be contained "IN" the map.

If the number of statements following the for/if is single you don't have to use curly braces.

But if the number of statements is more than one, then you need to use curly braces. If you don't use curly brace in this scenario, then the first line will execute only in the context of for/if, other lines will execute at all times.

Example:

if(condition){ // Not required
    statement1;
} // Not required

if(condition) { // Required
    statement1;
    statement2;
} // Required

if(condition)
statement1; // This will execute if condition satisfies
statement2; // This will execute anyway

for(iteration condition){ // Not required
    statement1;
} // Not required

for(iteration condition) { // Required
    statement1;
    statement2;
} // Required

for(iteration condition)
statement1; // This will execute for n times
statement2; // This will execute only once

Hope it helps to understand.