Write a fetchCol() Sql query in Magento 2.

Write a Mysql fetchCol() query using Magento standard way without using the Model file.

To Fetches the first column of all SQL result rows as an array.

Return Type: fetchCol() always returns as an array.

Base Definition:

/**
 * @param string|\Magento\Framework\DB\Select $sql An SQL SELECT statement.
 * @param mixed $bind Data to bind into SELECT placeholders.
 * @return array
 */
 public function fetchCol($sql, $bind = []);

Let’s write a query from the sales_order table to accomplish the fetchCol() query operation.

<?php
namespace Path\To\Class;

use Magento\Framework\App\ResourceConnection;

class FetchCol {

    const ORDER_TABLE = 'sales_order';

    private ResourceConnection $resourceConnection;

    public function __construct(
        ResourceConnection $resourceConnection
    ) {
        $this->resourceConnection = $resourceConnection;
    }

    /**
     * insert Sql Query
     *
     * @return string[]
     */
    public function fetchColQuery(): array
    {
        $connection  = $this->resourceConnection->getConnection();
        $tableName = $connection->getTableName(self::ORDER_TABLE);

        $query = $connection->select()
            ->from($tableName,['entity_id','status'])
            ->where('status = ?', 'pending');

        return $connection->fetchCol($query);
    }
}

Output:
An Array result with value is entity_id of sales_order table.

Array
(
    [1] => 15
    [2] => 17
    [3] => 19
    ....
)

15, 17, and 19 are the entity_id with pending status.