$wpdb->get_results(...) returns empty array despite correct query

Use $wpdb->show_errors( true ) before the query and see what error comes back.

My problem (because I've experienced the same thing) was that I should use $wpdb->posts instead of wp_posts inside the query. The prefix of the tables can change from WP installation to installation or even in the same installation depending on the time (e.g. an admin can change the prefix whenever he likes). So, one should write the query like that:

$query = "select <stuff here> from $wpdb->posts where <stuff here>";

I was troubleshooting my script that produced an empty array in WordPress with a seemingly healthy MySQL string. It turned out that, unlike the person who asked the question, I used $wpdb->prepare(), but the solution I found may still be useful to someone who comes across this Q&A (as I have).

This string, AND aki.type LIKE '%thing%'", contains percentage wilcards (%) that indicate that we're looking for aki.type cell that contains the thing as part of its value.

$wpdb->prepare converts them into hashes, and the MySQL server gets something like this: AND aki.type LIKE '{399038939300d2c307c53b29a166ee90101da7b2aba7978a898694010bf1bfe6}thing{399038939300d2c307c53b29a166ee90101da7b2aba7978a898694010bf1bfe6}'. Naturally, one would get no results!

The wpdb::prepare page reads, 'Literal percentage signs (%) in the query string must be written as %%. Percentage wildcards (for example, to use in LIKE syntax) must be passed via a substitution argument containing the complete LIKE string, these cannot be inserted directly in the query string. Also see wpdb::esc_like().'

So, the LIKE part must be prepared like this (example from the wpdb::esc_like() page):

$wild = '%';
$find = 'only 43% of planets';
$like = $wild . $wpdb->esc_like( $find ) . $wild;
$sql  = $wpdb->prepare( "SELECT * FROM $wpdb->posts WHERE post_content LIKE %s", $like );

Where $wpdb->esc_like() should go before $wpdb->prepare().

And if one wants to replace percentage wildcard placeholders manually, they should use $wpdb->remove_placeholder_escape() after $wpdb->prepare().

May be a bit late, but you are missing the "prepare" call necessary in wordpress:

$Table_Name    = $wpdb->prefix.'Your Table Name';
$sql_query     = $wpdb->prepare("SELECT * FROM $Table_Name WHERE aki.keyID=%d AND akb.someID=%d AND aki.type like %s", akb.keyID, ac.someID, ''%thing%');
$result        = $wpdb->query( $sql_query ); 

Multiple results can be found in the array.

I had the same issue a, a good sql statement, no errors, but no results using wpdb.

The fix for me was a matter of having the correct collation/character encoding (e.g. COLLATE 'utf8_general_ci') for my table.

I had a query that worked, the table was replaced with a dump from an other server (and older server) and my query stopped working. It returned NULL or 0 even thought running the query in the database returned the proper results.

I create the tables both ways (a couple times because I did not believe it), ran the same data insert script, and did not change my PHP/wpdb code.

When the table was created with this this, my query did NOT work.

CREATE TABLE IF NOT EXISTS `vf_user` (
  `id` mediumint(9) NOT NULL AUTO_INCREMENT,
  `email` varchar(50) NOT NULL,
  `full_name` varchar(25) NOT NULL,
  `password` varchar(25) NOT NULL,
  `role` enum('user','tester','admin') NOT NULL DEFAULT 'user',
  `phone` varchar(25) NOT NULL,
  `last_modified` timestamp NOT NULL DEFAULT current_timestamp() ON UPDATE current_timestamp(),
  `user_activation_key` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE,
  UNIQUE KEY `email` (`email`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=325 DEFAULT CHARSET=utf8 COMMENT='users table';

When it was created with this, my query did return results

CREATE TABLE `vf_user` (
    `id` MEDIUMINT(9) NOT NULL AUTO_INCREMENT,
    `email` VARCHAR(50) NOT NULL COLLATE 'utf8_general_ci',
    `full_name` VARCHAR(25) NOT NULL COLLATE 'utf8_general_ci',
    `password` VARCHAR(25) NOT NULL COLLATE 'utf8_general_ci',
    `role` ENUM('user','tester','admin') NOT NULL DEFAULT 'user' COLLATE 'utf8_general_ci',
    `phone` VARCHAR(25) NOT NULL COLLATE 'utf8_general_ci',
    `last_modified` TIMESTAMP NOT NULL DEFAULT current_timestamp() ON UPDATE current_timestamp(),
    `user_activation_key` VARCHAR(100) NULL DEFAULT NULL COLLATE 'utf8_general_ci',
    PRIMARY KEY (`id`) USING BTREE,
    UNIQUE INDEX `email` (`email`) USING BTREE
)
COMMENT='users with access to the virtual festival'
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=325
;

Wpdb->get_results function from wordpress returns the result if successful otherwise it will return null. There can be many reasons if a query get failed.Refer in-depth article on debugging get_results() returning empty results here

Although you can use functions like wpdb->show_error() to check what was the last error after executing the sql query. sometimes this error returns empty then try to use wpdb->last_query to check the final query that get formed.