MariaDB 10.2

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CREATE TABLE `phone_directory` (
  `lead_id` INT(9) UNSIGNED NOT NULL AUTO_INCREMENT,
  `list_id` BIGINT(14) UNSIGNED DEFAULT NULL,
  `gmt_offset_now` DECIMAL(4,2) DEFAULT '0.00',
  `first_name` VARCHAR(30) DEFAULT NULL,
  `middle_initial` CHAR(1) DEFAULT NULL,
  `last_name` VARCHAR(30) DEFAULT NULL,
  `address1` VARCHAR(100) DEFAULT NULL,
  `address2` VARCHAR(100) DEFAULT NULL,
  `address3` VARCHAR(100) DEFAULT NULL,
  `city` VARCHAR(50) DEFAULT NULL,
  `state` CHAR(2) DEFAULT NULL,
  `postal_code` VARCHAR(10) DEFAULT NULL,
  `email` VARCHAR(70) DEFAULT NULL,
  `fax_number` VARCHAR(255) DEFAULT NULL,
  `manager_name` VARCHAR(255) DEFAULT NULL,
  `status` VARCHAR(6) DEFAULT NULL,
  PRIMARY KEY (`lead_id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;
CREATE TABLE phones
(
    lead_id INT(9) NOT NULL REFERENCES phone_directory(lead_id),
    priority smallint NOT NULL,  -- If you need to give them priorities (1, 2, 3, ...), or sort them
    phone VARCHAR(12) NOT NULL COLLATE latin1_bin,  
    CONSTRAINT unique_phones UNIQUE(phone), -- You don't want repetead telephones. This enforces it.
    PRIMARY KEY(phone, lead_id)  -- Covering index + clustering... for the sake of efficiency
) ;
EXPLAIN
SELECT lead_id FROM phones WHERE phone IN ('315XXXXXXX','0315XXXXXXX');


id
select_type
table
type
possible_keys
key
key_len
ref
rows
Extra


1
SIMPLE
phones
index
PRIMARY,unique_phones
PRIMARY
18
null
1
Using where; Using index