each.

data "aws_subnets" "all_db" {
 filter {
   name   = "vpc-id"
   values = [var.vpc_id]
 }

 tags = {
   Tier = "DB"
 }
} 

data "aws_subnet" "all_db" {
 for_each = toset(data.aws_subnets.all_db.ids)
 id       = each.value
}

data "aws_subnets" "all_app" {
 filter {
   name   = "vpc-id"
   values = [var.vpc_id]
 }

 tags = {
   Tier = "WebApp"
 }
}

data "aws_subnet" "all_app" {
 for_each = toset(data.aws_subnets.all_app.ids)
 id       = each.value
}

See also

• Data block: data., toset, each.
• for_each, each., Terraform: dynamic_blocks