Apex - SOQL

This is Salesforce Object Query Language designed to work with SFDC Database. It can search a record on a given criterion only in single sObject.
Like SOSL, it cannot search across multiple objects but it does support nested queries.
Consider our ongoing example of Chemical Company. Suppose, we need a list of records which are created today and whose customer name is not 'test'. In this case, we will have to use the SOQL query as given below −
// fetching the Records via SOQL
List<apex_invoice__c> InvoiceList = new List<apex_invoice__c>();
InvoiceList = [SELECT Id, Name, APEX_Customer__r.Name, APEX_Status__c FROM
   APEX_Invoice__c WHERE createdDate = today AND APEX_Customer__r.Name != 'Test'];
// SOQL query for given criteria

// Printing the fetched records
System.debug('We have total '+InvoiceList.size()+' Records in List');

for (APEX_Invoice__c objInvoice: InvoiceList) {
   System.debug('Record Value is '+objInvoice); 
   // Printing the Record fetched
}
You can run the SOQL query via the Query Editor in the Developer console as shown below.
Run the query given below in the Developer Console. Search for the Invoice records created today.
SELECT Id, Name, APEX_Customer__r.Name, APEX_Status__c FROM APEX_Invoice__c
   WHERE createdDate = today
You must select the fields for which you need the values, otherwise, it can throw run time errors.
This is one of the most important parts in SFDC as many times we need to traverse through the parent child object relationship
Also, there may be cases when you need to insert two associated objects records in Database. For example, Invoice object has relationship with the Customer object and hence one Customer can have many invoices.
Suppose, you are creating the invoice and then you need to relate this invoice to Customer. You can use the following code for this functionality −
// Now create the invoice record and relate it with the Customer object
// Before executing this, please create a Customer Records with Name 'Customer
// Creation Test'
APEX_Invoice__c objInvoice = new APEX_Invoice__c();

// Relating Invoice to customer via id field of Customer object
objInvoice.APEX_Customer__c = [SELECT id FROM APEX_Customer__c WHERE Name =
   'Customer Creation Test' LIMIT 1].id;
objInvoice.APEX_Status__c = 'Pending';
insert objInvoice;  //Creating Invoice
System.debug('Newly Created Invoice'+objInvoice);  //Newly created invoice
Execute this code snippet in the Developer Console. Once executed, copy the Id of invoice from the Developer console and then open the same in SFDC as shown below. You can see that the Parent record has already been assigned to Invoice record as shown below.
Let us now consider an example wherein, all the invoices related to particular customer record need to be in one place. For this, you must know the child relationship name. To see the child relationship name, go to the field detail page on the child object and check the "Child Relationship" value. In our example, it is invoices appended by __r at the end.
In this example, we will need to set up data, create a customer with name as 'ABC Customer' record and then add 3 invoices to that customer.
Now, we will fetch the invoices the Customer 'ABC Customer' has. Following is the query for the same −
// Fetching Child Records using SOQL
List<apex_customer__c> ListCustomers = [SELECT Name, Id, 
   (SELECT id, Name FROM Invoices__r) FROM APEX_Customer__c WHERE Name = 'ABC Customer'];

// Query for fetching the Child records along with Parent
System.debug('ListCustomers '+ListCustomers); // Parent Record

List<apex_invoice__c> ListOfInvoices = ListCustomers[0].Invoices__r;
// By this notation, you could fetch the child records and save it in List
System.debug('ListOfInvoices values of Child '+ListOfInvoices);
// Child records
You can see the Record values in the Debug logs.
Suppose, you need to fetch the Customer Name of Invoice the creation date of which is today, then you can use the query given below for the same −
Fetch the Parent record's value along with the child object.
// Fetching Parent Record Field value using SOQL
List<apex_invoice__c> ListOfInvoicesWithCustomerName = new List<apex_invoice__c>();
ListOfInvoicesWithCustomerName = [SELECT Name, id, APEX_Customer__r.Name 
   FROM APEX_Invoice__c LIMIT 10];

// Fetching the Parent record's values
for (APEX_Invoice__c objInv: ListOfInvoicesWithCustomerName) {
   System.debug('Invoice Customer Name is '+objInv.APEX_Customer__r.Name);
   // Will print the values, all the Customer Records will be printed
}
Here we have used the notation APEX_Customer__r.Name, where APEX_Customer__r is parent relationship name, here you have to append the __r at the end of the Parent field and then you can fetch the parent field value.
SOQL does have aggregate function as we have in SQL. Aggregate functions allow us to roll up and summarize the data. Let us now understand the function in detail.
Suppose, you wanted to know that what is the average revenue we are getting from Customer 'ABC Customer', then you can use this function to take up the average.
// Getting Average of all the invoices for a Perticular Customer
AggregateResult[] groupedResults = [SELECT
   AVG(APEX_Amount_Paid__c)averageAmount FROM APEX_Invoice__c WHERE
   APEX_Customer__r.Name = 'ABC Customer'];
Object avgPaidAmount = groupedResults[0].get('averageAmount');
System.debug('Total Average Amount Received From Customer ABC is '+avgPaidAmount);
Check the output in Debug logs. Note that any query that includes an aggregate function returns its results in an array of AggregateResult objects. AggregateResult is a readonly sObject and is only used for query results. It is useful when we need to generate the Report on Large data.
There are other aggregate functions as well which you can be used to perform data
summary.
MIN() − This can be used to find the minimum value
MAX() − This can be used to find the maximum value.
You can use the Apex variable in SOQL query to fetch the desired results. Apex variables
can be referenced by the Colon (:) notation.
// Apex Variable Reference
String CustomerName = 'ABC Customer';
List<apex_customer__c> ListCustomer = [SELECT Id, Name FROM APEX_Customer__c
   WHERE Name = :CustomerName];

// Query Using Apex variable
System.debug('ListCustomer Name'+ListCustomer); // Customer Name

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