General Example Rules

If you want to prevent changes made once a certain picklist value has been chosen (then only admins are allowed to change that specific picklist value). In the following case, this is done with a listing status preventing anyone from changing it once it is "Published", but could easily be applied to other objects, too:

AND (
ISPICKVAL(pba__Status__c, "Published"),
OR (
ISNEW(),
ISCHANGED ( pba__Status__c )
),
AND (
$Profile.Name <> "PB Administrator",
$Profile.Name <> "PB Office Admin"
))

In the linked article below Salesforce explains how to disallow making changes to a "closed" closing (Salesforce calls this "Opportunity", simply replace that with "Closing" when reading it):

https://help.salesforce.com/articleView?id=000005286&type=1

Picklist validation with a Date field to be mandatory

Situation: You want to make sure to capture the closing date the latest when the closing status is changed to "Closed".
Solution: To do that, the user agent must populate the closing date if this validation rule is in place:

AND( 
ISPICKVAL(Closing_Status__c, 'Closed'), 
ISBLANK(ClosingDate__c) 
)

Picklist Validation with another Picklist field to be mandatory

Situation: when a contact becomes an active contact, you want to be sure that the contact type has been set.
Solution: The following validation rule will ask the user to select at least one contact type from the contact type multipicklist before he can change the contact stage to "active":

AND( 
ISPICKVAL(Stage__c , 'Active'), 
ISNULL(ContactType__c))

Note: Picklists and Multipicklists can be tricky to use in formulas and you might need read Salesforce help (see below) or search how others did it.

You can use validation rules on other records as well, Salesforce has some great ideas here: https://developer.salesforce.com/docs/atlas.en-us.usefulValidationRules.meta/usefulValidationRules/fields_useful_field_validation_formulas.htm
and in this PDF "EXAMPLES OF VALIDATION RULES" here https://resources.docs.salesforce.com/206/latest/en-us/sfdc/pdf/salesforce_useful_validation_formulas.pdf

Preventing Ownership-Change: A user should not be able to give his record to someone else

On objects where you want to prevent that a user passes over a record to someone else, you could prevent this with a Validation Rule that checks whether the user is an Admin or Superuser by profile and if not, disallows the change of ownership away from the user.

AND(ISCHANGED(OwnerId), 
OR($Profile.Name <> 'PB Administrator'),
($Profile.Name <> 'PB Superuser'))

This typically would go hand-in-hand with not having the permission in the profile to "Transfer Records" which ensures they cannot take a record from someone else either.

Listing / Portal Example Rules

Portal Mandatory Fields Validation

AND( 
OR( 
ISBLANK(pba__Broker_s_Listing_ ID__c),
ISBLANK(Displayed_Address__c),
ISPICKVAL(pba__PropertyType__ c,""),
ISBLANK( pba__ListingPrice_pb__c ), 
ISBLANK( Summary__c ), 
ISBLANK( pba__Bedrooms_pb__c ), 
ISBLANK( pba__Description_pb__c ), 
ISBLANK( pba__City_pb__c ), 
ISBLANK( pba__PostalCode_pb__c ), 
ISBLANK(pba__Address_pb__c)), 
OR( 
TEXT(pba__Status__c) = "Active", 
TEXT(pba__Status__c) = "Any Other Custom Status That Should Be Allowed To Go Online", 
TEXT(pba__Status__c) = "Under Offer" 
))

AND( 
ISBLANK( pba__ListingPrice_pb__c ), 
OR(ISPICKVAL(pba__Status__c, "Available"),
ISPICKVAL(pba__Status__c, "Active"),
ISPICKVAL(pba__Status__c, "Pending")
))

This then works the same way for other fields as well, like Property Type or Property Sub Type and so on - just consider which field you want this for and which statuses it should check:

Energy Certificate Information

AND(
OR(
ISPICKVAL( pba_espfields__Energy__c, "A"),
ISPICKVAL( pba_espfields__Energy__c, "B"),
ISPICKVAL( pba_espfields__Energy__c, "C"),
ISPICKVAL( pba_espfields__Energy__c, "D"),
ISPICKVAL( pba_espfields__Energy__c, "E"),
ISPICKVAL( pba_espfields__Energy__c, "F"),
ISPICKVAL( pba_espfields__Energy__c, "G"),
ISPICKVAL( pba_espfields__Energy__c,""),
ISPICKVAL( EnvironmentImpactRating__c,""),
ISPICKVAL( EnvironmentImpactRating__c, "A"),
ISPICKVAL( EnvironmentImpactRating__c, "B"),
ISPICKVAL( EnvironmentImpactRating__c, "C"),
ISPICKVAL( EnvironmentImpactRating__c, "D"),
ISPICKVAL( EnvironmentImpactRating__c, "E"),
ISPICKVAL( EnvironmentImpactRating__c, "F"),
ISPICKVAL( EnvironmentImpactRating__c, "G")),
OR(
ISBLANK (pba_espfields__Energy_Performance__c),
ISBLANK (EnvironmentImpactValue__c)),
ISPICKVAL(pba__Status__c, "Active")
)

Field Length Limitation Validation

And of course, you can add more fields or criteria to enhance listing quality, e.g. limit the amount of characters allowed in the address field by using:

LEN(Addressfieldname__c) > 60

This will work with all fields where you want to ensure a certain amount of characters is being added. You should always consider enhancing the rule to perhaps only trigger under certain conditions like "Status" = "Active" to allow users to input or skip the field as long as not necessary/required.

Postal / ZIP Code Validation Rule Examples

AND(OR(

ISPICKVAL(pba__Status__c, "Active"),
ISPICKVAL(pba__Status__c, "Pending")),

(NOT(OR(

REGEX( pba__PostalCode_pb__c , "[A-Z]\\d\\s\\d[A-Z]{2}"),

REGEX( pba__PostalCode_pb__c , "[A-Z]\\d\\d\\s\\d[A-Z]{2}"),

REGEX( pba__PostalCode_pb__c , "[A-Z]{2}\\d\\s\\d[A-Z]{2}"),

REGEX( pba__PostalCode_pb__c , "[A-Z]{2}\\d\\d\\s\\d[A-Z]{2}"),

REGEX( pba__PostalCode_pb__c , "[A-Z]{2}\\d[A-Z]\\s\\d[A-Z]{2}"),

REGEX( pba__PostalCode_pb__c , "[A-Z]\\d[A-Z]\\s\\d[A-Z]{2}")

)
)
)
)

2. Sometimes only certain postal code formats are permitted. To prevent a user from adding something else, you can check what was entered with regular expressions. In this example, this is what Idealista will expect to see: a format like AD123 or 12345 or 1234-123, so 

'^[0-9]{5}$|^AD[0-9]{3}$|^[0-9]{4}(-[0-9]{3})?$'

Images/Media Example Rules

1. Create a new field of type "Roll-Up Summary" on the "Property" object called "PropertyMediaCount" counting "PropertyMedia". After creating it, it should look somewhat like this:
   
   
2. Then switch to the "Listing" object in Object Manager and add this validation rule (note: if you used a different field name for the counting field on Property, please adjust accordingly):

   AND(
   OR(TEXT(pba__Status__c) = "Active"),
   OR(pba__Property__r.PropertyMediaCount__c = 0))

   
   
   This will ensure that there must be at least one image (count greater than 0) when the listing's status becomes "Active". Obviously you can add your own "active" statuses as explained in other rules in this article.

Recommended Addition: You can even go a step further and have different Roll-Up Summary fields on the "Property" object counting different cases: when creating a roll-up summary field, you can also add criteria to what the field should count.

If you choose e.g. that "Portal Feed equals true", then it will only count the amount of images that have been added specifically to be published to portals. If the validation rule then checks for "greater than 0" on that field, it will basically check, if not only images are present, but also ensure that at least one has been added to be published to portals:


Of course you can do this with "website" or "exposé" as well and you could go yet a step beyond and check whether the mime type is an image (and not a document or pdf) and vice versa.  So there are multiple combinations you could use this with criteria on the field.

Contact Example Rules

AND(
ISNEW(),
ISBLANK(Phone),
ISBLANK(Email))

AND(ISBLANK(Phone),
ISBLANK(Email))

AND(ISBLANK(Phone),
ISBLANK(Email),
ISPICKVAL(Stage__c, "Active"))

AND(ISBLANK(Phone),
ISBLANK( MobilePhone),
ISBLANK(Email),
ISPICKVAL( pba__Stage_pb__c , "Contacted Lead"))

Closing Example Rules

AND 
( 
ISNEW(), 
NOT(pba__Listing__r.pba__SystemHasAcceptedOffer__c = FALSE), 
NOT(pba__Listing__r.pba__SystemHasExclusiveOffer__c = FALSE), 
NOT( ISPICKVAL(Closing_Status__c, "Canceled" )))