Source: http://www.talkapex.com/2010/06/conditional-validations-in-apex

There may be some instances when you have validations on your APEX page that you only want to run if all the other validations have passed. A good example of this is if you have 2 fields, FROM_DATE and TO_DATE and have the following validations:
- V1: Ensure FROM_DATE is not null and is a valid date
- V2: Ensure TO_DATE is not null and is a valid date
- V3: Ensure FROM_DATE < TO_DATE

In this example you only want the last validation to run if the first 2 pass (i.e. both FROM_DATE and TO_DATE are valid dates). Currently there's no declarative way of doing this. i.e. validations don't support declarative dependencies.

A simple trick to get around this issue is to put a condition in some validations to only run if all the other validations pass. This may not work in all situations, however it should help you in some.

In the example about, modify the 3rd validation (V3) and set the condition to the following:
Condition Type: PL/SQL Expression
Expression 1: apex_application.g_inline_validation_error_cnt = 0

Now V3 will only run if all previous validations pass.

Validations are usually executed during a page submit, and nice error messages are shown to the user.
However, if you want to validate and process data using a Dynamic Action, you need some JavaScript.

In the example below I have implemented a "Value Required" validation, with some code to get the error message from Apex and the item label.
Try typing both nothing and some text in the item below and hitting the button.

Some useful links:

• https://stackoverflow.com/questions/63891864/put-validation-on-page-item-through-dynamic-action
• https://docs.oracle.com/en/database/oracle/apex/24.1/aexjs/apex.message.html#.showErrors
• http://apex-smb.blogspot.com/2014/01/stopping-dynamic-actions-using.html

Item

Identification
	Sequence	10
	Name	P1019_ITEM_01
	Display As	Text Field
User Interface
	Region	Validations for Dynamic Actions

Button

Name
	Button Name	VALIDATIONS
	Text Label / Alt	Validations
Displayed
	Sequence	20
	Display in Region	Validations for Dynamic Actions
Behavior
	Action	Defined by Dynamic Action
	Execute Validations	Yes

Dynamic Action

Identification
	Name	Validation DA
Execution Options
	Sequence	10
When
	Event	Click
	Selection Type	Button
	Button	VALIDATIONS
Advanced
	Event Scope	Static
True Action
Identification
	Action	Execute JavaScript Code
Settings
	Code	apex.message.clearErrors(); var itemName = "P1019_ITEM_01"; if ($v(itemName) == '') { var itemLabel = $("#" + itemName + "_LABEL").text().trim(); var errMsg = apex.lang.getMessage("APEX.PAGE_ITEM_IS_REQUIRED").replace("#LABEL#", itemLabel); apex.message.showErrors( { type: "error", location: ["inline", "page"], pageItem: itemName, message: errMsg, unsafe: false } ); return false; /* Stops all following Dynamic Actions from running */ }
Execution Options
	Sequence	40
	Fire On Initialization	No
True Action
Identification
	Action	Alert
Settings
	Text	You got past the validation
Execution Options
	Sequence	60
	Fire On Initialization	No