SOQL For Loops Versus Standard SOQL Queries [IMPORTANT]

SOQL for loops differ from standard SOQL statements because of the method they use to retrieve sObjects. While the standard queries discussed in SOQL and SOSL Queries can retrieve either the count of a query or a number of object records, SOQL for loops retrieve all sObjects, using efficient chunking with calls to the query and queryMore methods of the SOAP API. Developers should always use a SOQL for loop to process query results that return many records, to avoid the limit on heap size.

Note that queries including an aggregate function don't support queryMore. A run-time exception occurs if you use a query containing an aggregate function that returns more than 2,000 rows in a for loop.

SOQL For Loop Formats

SOQL for loops can process records one at a time using a single sObject variable, or in batches of 200 sObjects at a time using an sObject list:

• The single sObject format executes the for loop's <code_block> once per sObject record. Consequently, it is easy to understand and use, but is grossly inefficient if you want to use data manipulation language (DML) statements within the for loop body. Each DML statement ends up processing only one sObject at a time.
• The sObject list format executes the for loop's <code_block> once per list of 200 sObjects. Consequently, it is a little more difficult to understand and use, but is the optimal choice if you need to use DML statements within the for loop body. Each DML statement can bulk process a list of sObjects at a time.

For example, the following code illustrates the difference between the two types of SOQL query for loops:

01	// Create a savepoint because the data should not be committed to the database

02	Savepoint sp = Database.setSavepoint();

03

04	insert new Account[]{new Account(Name = 'yyy'),

05	new Account(Name = 'yyy'),

06	new Account(Name = 'yyy')};

07

08	// The single sObject format executes the for loop once per returned record

09	Integer i = 0;

10	for (Account tmp : [SELECT Id FROM Account WHERE Name = 'yyy']) {

11

i++;

12

}

13	System.assert(i == 3); // Since there were three accounts named 'yyy' in the

14	// database, the loop executed three times

15

16	// The sObject list format executes the for loop once per returned batch

17	// of records

18

i = 0;

19	Integer j;

20	for (Account[] tmp : [SELECT Id FROM Account WHERE Name = 'yyy']) {

21	j = tmp.size();

22

i++;

23

}

24	System.assert(j == 3); // The list should have contained the three accounts

25	// named 'yyy'

26	System.assert(i == 1); // Since a single batch can hold up to 200 records and,

27	// only three records should have been returned, the

28	// loop should have executed only once

29

30	// Revert the database to the original state

31	Database.rollback(sp);

• The break and continue keywords can be used in both types of inline query for loop formats. When using the sObject list format, continue skips to the next list of sObjects.
• DML statements can only process up to 10,000 records at a time, and sObject list for loops process records in batches of 200. Consequently, if you are inserting, updating, or deleting more than one record per returned record in an sObject list for loop, it is possible to encounter runtime limit errors. See Execution Governors and Limits.
• You might get a QueryException in a SOQL for loop with the message Aggregate query has too many rows for direct assignment, use FOR loop. This exception is sometimes thrown when accessing a large set of child records (200 or more) of a retrieved sObject inside the loop, or when getting the size of such a record set. For example, the query in the following SOQL for loop retrieves child contacts for a particular account. If this account contains more than 200 child contacts, the statements in the for loop cause an exception.

  1	for (Account acct : [SELECT Id, Name, (SELECT Id, Name FROM Contacts)

  2	FROM Account WHERE Id IN ('<ID value>')]) {

  3	List<Contact> contactList = acct.Contacts; // Causes an error

  4	Integer count = acct.Contacts.size(); // Causes an error

  5

  }

  To avoid getting this exception, use a for loop to iterate over the child records, as follows.

  1	for (Account acct : [SELECT Id, Name, (SELECT Id, Name FROM Contacts)

  2	FROM Account WHERE Id IN ('<ID value>')]) {

  3	Integer count=0;

  4	for (Contact c : acct.Contacts) {

  5

  count++;

  6

  }

  7

  }