What can we use for exist query in magento2?

Question

We have two approaches to find if the record exists in database or not

First

$select = $this->bookResource->getConnection()->select()
->distinct()
->from(
    $this->bookResource->getMainTable(),
    "author_id"
)
->where("author_id = ?", $authorId);
if ($this->bookResource->getConnection()->fetchOne($select)) {
      //record exist
}

second

$bookCollection->addFieldToSelect('author_id')
    ->addFieldToFilter("author_id", $authorId);
if ($bookCollection->getSize()){
    // record exist
}

which one to use and why

Answer 1

I would use the first one but with a limit.

$select = $this->bookResource->getConnection()->select()
->distinct()
->from(
    $this->bookResource->getMainTable(),
    "author_id"
)
->where("author_id = ?", $authorId)
->limit(1);

This will run a query like SELECT DISTINCT(..) FROM ... WHERE author_id = ... LIMIT 1.

Without the limit, if there are multiple rows matching your criteria, all will be retrieved from the db and one will be returned.
The second approach, with getSize runs a query like SELECT COUNT(*) from ... where.... it may run slow if you have a lot of records.

But if you want to respect the Law Of Demeter you can use a combination of the 2.

$bookCollection->addFieldToSelect('author_id')
    ->addFieldToFilter("author_id", $authorId)
    ->setPageSize(1)
    ->setCurPage(1);
$book = $bookCollection->getFisrtItem();
if ($book->getId()) {
    //a row exists
} else {
    //there is no row for the selected author.
}

Answer 2

class testIfExist
{
/**
 * @var ResourceConnection
 */
private $resourceConnection;

/**
 * @param ResourceConnection $resourceConnection
 */
public function __construct (
    ResourceConnection $resourceConnection
) {
    $this->resourceConnection = $resourceConnection;
}

/**
 * @param string $pageId
 * @return bool
 */
public function execute(string $id): bool
{
    $connection = $this->resourceConnection->getConnection();

    $tableName = $this->resourceConnection->getTableName('table-name-here');

    $qry = $connection->select()
        ->from($tableName)
        ->where('entity_id_of_table = ?', $id);

    return (bool)$connection->fetchOne($qry);
}
}

easy class to call

Cheers