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Malli GMalli G 

can we make dml operation and soql on trigger.old in beforeupdate event .if we can do give someexample

Mahesh DMahesh D
Hi Malli,

In general we shouldn't perform any DML operations in Before Triggers. DML operations will be suggestable to do it in After Triggers.

SOQL Queries we can write in Both Before and After Update triggers.

Please check the below example:

Here we are Iterating through all the input Account records in both insert and update scenario and populating the Industry field with the value: Cloud Computing
trigger SetDefaultAccountValues on Account (before insert, before update) {  
    for (Account oAccount : {
        oAccount.Industry = ‘Cloud Computing’;
Another example here:
trigger myAccountTrigger on Account(before delete, before insert, before update, 
                                    after delete, after insert, after update) {
if (Trigger.isBefore) {
    if (Trigger.isDelete) {

        // In a before delete trigger, the trigger accesses the records that will be
        // deleted with the Trigger.old list.
        for (Account a : Trigger.old) {
            if ( != 'okToDelete') {
                a.addError('You can\'t delete this record!');
    } else {

    // In before insert or before update triggers, the trigger accesses the new records
    // with the list.
        for (Account a : {
            if ( == 'bad') {
      'Bad name');
    if (Trigger.isInsert) {
        for (Account a : {
            System.assertEquals('xxx', a.accountNumber); 
            System.assertEquals('industry', a.industry); 
            System.assertEquals(100, a.numberofemployees);
            System.assertEquals(100.0, a.annualrevenue);
            a.accountNumber = 'yyy';

// If the trigger is not a before trigger, it must be an after trigger.
} else {
    if (Trigger.isInsert) {
        List<Contact> contacts = new List<Contact>();
        for (Account a : {        
            if(a.Name == 'makeContact') {
                contacts.add(new Contact (LastName = a.Name,
                                          AccountId = a.Id));
      insert contacts;

Best Practices of writing the Trigger

1) One Trigger Per Object
A single Apex Trigger is all you need for one particular object. If you develop multiple Triggers for a single object, you have no way of controlling the order of execution if those Triggers can run in the same contexts

2) Logic-less Triggers
If you write methods in your Triggers, those can’t be exposed for test purposes. You also can’t expose logic to be re-used anywhere else in your org.

3) Context-Specific Handler Methods
Create context-specific handler methods in Trigger handlers

4) Bulkify your Code
Bulkifying Apex code refers to the concept of making sure the code properly handles more than one record at a time.

5) Avoid SOQL Queries or DML statements inside FOR Loops
An individual Apex request gets a maximum of 100 SOQL queries before exceeding that governor limit. So if this trigger is invoked by a batch of more than 100 Account records, the governor limit will throw a runtime exception

6) Using Collections, Streamlining Queries, and Efficient For Loops
It is important to use Apex Collections to efficiently query data and store the data in memory. A combination of using collections and streamlining SOQL queries can substantially help writing efficient Apex code and avoid governor limits

7) Querying Large Data Sets
The total number of records that can be returned by SOQL queries in a request is 50,000. If returning a large set of queries causes you to exceed your heap limit, then a SOQL query for loop must be used instead. It can process multiple batches of records through the use of internal calls to query and queryMore

8) Use @future Appropriately
It is critical to write your Apex code to efficiently handle bulk or many records at a time. This is also true for asynchronous Apex methods (those annotated with the @future keyword). The differences between synchronous and asynchronous Apex can be found

9) Avoid Hardcoding IDs
When deploying Apex code between sandbox and production environments, or installing AppExchange packages, it is essential to avoid hardcoding IDs in the Apex code. By doing so, if the record IDs change between environments, the logic can dynamically identify the proper data to operate against and not fail.

Few more Best Practices for Triggers
There should only be one trigger for each object.
Avoid complex logic in triggers. To simplify testing and resuse, triggers should delegate to Apex classes which contain the actual execution logic. See Mike Leach's excellent trigger template for more info.
Bulkify any "helper" classes and/or methods.
Trigers should be "bulkified" and be able to process up to 200 records for each call.
Execute DML statements using collections instead of individual records per DML statement.
Use Collections in SOQL "WHERE" clauses to retrieve all records back in single query
Use a consistent naming convention including the object name (e.g., AccountTrigger)

Also go through the below links:

Please do let me know if it helps you.