Build a Bulk-Safe Trigger

trigger LeaveRequestTrigger on Leave_Request__c (before insert) {
    new LeaveRequestTriggerHandler().beforeInsert(Trigger.new);
}

1. trigger LeaveRequestTrigger on Leave_Request__c (before insert) { — exactly Module 24's thin-trigger pattern — before insert, since overlap validation must block the save (Module 26's addError() lesson), which only works in a before context.
2. new LeaveRequestTriggerHandler().beforeInsert(Trigger.new); — hands off to the handler.
3. } — closes the trigger.

public void beforeInsert(List<Leave_Request__c> newRequests) {
    Set<String> employeeNames = new Set<String>();
    for (Leave_Request__c req : newRequests) {
        employeeNames.add(req.Employee_Name__c);
    }

    List<Leave_Request__c> existingRequests = [
        SELECT Id, Employee_Name__c, Start_Date__c, End_Date__c
        FROM Leave_Request__c
        WHERE Employee_Name__c IN :employeeNames AND Status__c != 'Rejected'
    ];
    // ... validation logic uses existingRequests (next lessons) ...
}

1. public void beforeInsert(List<Leave_Request__c> newRequests) { — the method.
2. Set<String> employeeNames = new Set<String>(); — this is Module 16's Set deduplication.
3. for (Leave_Request__c req : newRequests) { — loops over the new requests, in memory.
4. employeeNames.add(req.Employee_Name__c); — collects every affected employee's name once.
5. } — closes the loop.
6. (blank line)
7. List<Leave_Request__c> existingRequests = [ — one upfront query.
8. SELECT Id, Employee_Name__c, Start_Date__c, End_Date__c — the fields needed.
9. FROM Leave_Request__c — the object.
10. WHERE Employee_Name__c IN :employeeNames AND Status__c != 'Rejected' — all of their existing, non-rejected requests in a single statement.
11. ]; — closes the query.
12. // ... validation logic uses existingRequests (next lessons) ... — the next step, before any per-record comparison starts.
13. } — closes the method.

// WRONG — a query per new request, exactly the anti-pattern from Module 25
for (Leave_Request__c req : newRequests) {
    List<Leave_Request__c> existing = [SELECT Id FROM Leave_Request__c WHERE Employee_Name__c = :req.Employee_Name__c];
    // ...
}

1. // WRONG — a query per new request, exactly the anti-pattern from Module 25 — the label says it all.
2. for (Leave_Request__c req : newRequests) { — loops over the new requests.
3. List<Leave_Request__c> existing = [SELECT Id FROM Leave_Request__c WHERE Employee_Name__c = :req.Employee_Name__c]; — querying inside the per-record loop would be the SOQL-in-a-loop problem from Module 25, at exactly the scale this feature is most likely to face — a bulk data load inserting hundreds of leave requests at once.
4. // ... — the rest of the loop body.
5. } — closes the loop.

Fetching everything needed before the loop begins is what keeps the actual comparison loop (next lesson) free of any database operation at all.