Vitaliy Orlov

$sql = mysqli_query($link, "SELECT * FROM reg WHERE name='$name' LIMIT 1");
if (!mysqli_num_rows($sql)) { // логин не найден
    echo 'неверный логин и пароль'; 
} else { // логин найден
    $result = mysqli_fetch_array($sql)
    if($result['password'] == $password){ // пароль верный
        echo 'логин и пароль верные';
    }else{ // пароль не верный
        echo 'неверный логин и пароль';
    }
}

$stmt = mysqli_prepare($link, "INSERT IGNORE INTO Users (a1, a2, a3, s1, s2) VALUES (?, ?, ?, ?, ?)");
mysqli_stmt_bind_param($stmt, 'sssss', $a1, $a2, $a3, $s1, $s2);

foreach($json['object'] as $item) {
  $a1 = $item['a1'];
  $a2 = $item['a2'];
  $a3 = $item['a3'];
  $s1 = $item['s1'];
  $s2 = $item['s2'];
  mysqli_stmt_execute($stmt);
}
  mysqli_stmt_close($stmt);
  mysqli_close($link);

-- высший приоритет
INTERVAL
BINARY, COLLATE
!
- (unary minus), ~ (unary bit inversion)
^
*, /, DIV, %, MOD
-, +
<<, >>
&
|
= (comparison), <=>, >=, >, <=, <, <>, !=, IS, LIKE, REGEXP, IN
BETWEEN, CASE, WHEN, THEN, ELSE
NOT
AND, &&
XOR
OR, ||
= (assignment), :=
-- низший приоритет

mysql> SELECT 1+2*3;
        -> 7
mysql> SELECT (1+2)*3;
        -> 9

`act_dt` > (NOW() - INTERVAL 30 DAY)
`act_dt` > NOW() - INTERVAL 30 DAY

(`act_dt` BETWEEN '12.12.2019' AND '15.15.2019') AND valid='on' 
`act_dt` BETWEEN '12.12.2019' AND '15.15.2019' AND valid='on'

product.id
product.title
product.seller_id

seller.id
seller.name
seller.cat_id

SELECT * FROM product 
LEFT JOIN seller ON product.seller_id = seller.id 
WHERE seller.cat_id=1

e_mail = '$e_mail' phone_number = '$phone_number' region

e_mail = '$e_mail', phone_number = '$phone_number', region

DROP PROCEDURE add_parent_id_to_city;

DELIMITER //
CREATE PROCEDURE add_parent_id_to_city ()
BEGIN
    DECLARE i INT DEFAULT 4700;
    WHILE (i > 0) DO
      SELECT 1+2 as result;
      SET i = i - 1;
    END WHILE;
END//

DELETE FROM b_iblock_section WHERE IBLOCK_ID=4 LIMIT 100

DELETE FROM b_iblock_section WHERE id IN ( 
	SELECT * FROM (
		SELECT ID FROM b_iblock_section WHERE IBLOCK_ID=4 LIMIT 0,100
	) as `id`
)

не пойму как мне сделать запрос на .. скажем 7 соединенных таблиц?

->orderBy([
    new \yii\db\Expression('FIELD (id, 220)'),
    'name' => SORT_ASC,
]);

....
unset($countries[220]);
$countries = [220 => 'Country Name'] + $countries;
return $countries;

UPDATE table1 t1 JOIN table2 t2 ON t1.id = t2.id
SET t1.col1 = t2.col1, t1.col2 = t2.col2

UPDATE t1 SET col1 = (SELECT col1 FROM t2 WHERE id = t1.t2_id)