MySQL foreign key not found even if it exists - mysql

Question

I have 2 tables 'open_invoices' and 'paid_invoices' with the below structures:
CREATE TABLE `open_invoices` (
`id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`unique_identifier` varchar(255) NOT NULL,
`insert_date` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
`cust_nbr` int(11) NOT NULL,
`location` varchar(10) NOT NULL,
`company` varchar(10) NOT NULL,
`invoice_nbr` int(11) NOT NULL,
`future_letter` varchar(1) NOT NULL,
`invoice_total` decimal(8,2) NOT NULL,
`invoice_payments` decimal(8,2) NOT NULL,
`record_type` int(11) NOT NULL,
`terms_desc` varchar(255) NOT NULL,
`due_date` varchar(8) NOT NULL,
`discount_date` varchar(8) NOT NULL,
`orig_disc_avail` decimal(8,2) NOT NULL,
`cust_po` varchar(10) NOT NULL,
`invoice_date` varchar(8) NOT NULL,
`status` int(11) NOT NULL,
`check_no` varchar(10) NOT NULL,
`last_pay_date` varchar(8) NOT NULL,
`as_of_date` varchar(8) NOT NULL,
`as_of_time` varchar(11) NOT NULL,
`remaining` varchar(1) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `unique_identifier` (`unique_identifier`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE `paid_invoices` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`date_change` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
`invoice_date` varchar(8) NOT NULL,
`unique_identifier` varchar(255) NOT NULL,
`amount_paid` decimal(8,2) NOT NULL,
`amount_left` decimal(8,2) NOT NULL,
`payment_type` varchar(2) NOT NULL,
`last4` varchar(255) NOT NULL,
`transac_id` varchar(255) NOT NULL,
`customer_id` varchar(255) NOT NULL,
`payment_P_F` varchar(1) NOT NULL,
PRIMARY KEY (`id`),
FOREIGN KEY (unique_identifier)
REFERENCES open_invoices (unique_identifier)
ON DELETE CASCADE
ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
I'm trying to insert a row in 'paid_invoices' like below:
INSERT INTO `paid_invoices` (`invoice_date`, `unique_identifier`, `amount_paid`, `amount_left`, `payment_type`, `last4`, `transac_id`, `customer_id`, `payment_P_F`) VALUES ('07/19/18', '126_89948_2576', 0, '37.45', '', '', '', '13776', 'P')
I get the below error:
Error Number: 1452
Cannot add or update a child row: a foreign key constraint fails (`accounts_receivables`.`paid_invoices`, CONSTRAINT `paid_invoices_ibfk_1` FOREIGN KEY (`unique_identifier`) REFERENCES `open_invoices` (`unique_identifier`))
INSERT INTO `paid_invoices` (`invoice_date`, `unique_identifier`, `amount_paid`, `amount_left`, `payment_type`, `last4`, `transac_id`, `customer_id`, `payment_P_F`) VALUES ('07/19/18', '126_89948_2576', 0, '37.45', '', '', '', '13776', 'P')
If I search on google, it says that the foreign key 126_89948_2576 was not found in the table 'open_invoices'.
When I do SELECT * FROM open_invoices WHERE id = 2576 I see the row with the foreign key 126_89948_2576 as shown in the picture below :
[![query 1][1]][1]
but when I do this query SELECT * FROM open_invoices WHERE unique_identifier = '126_89948_2576' I get no result see capture below:
[![query 2][2]][2]
I'm sure there is no extra blank space in the value saved for foreign key 126_89948_2576.
What is going on please? Is there a bug in the version of MySQL I'm using?
Server: Localhost via UNIX socket
Server type: MySQL
Server version: 5.6.40 - MySQL Community Server (GPL)
Protocol version: 10
Thanks.
[1]: https://i.stack.imgur.com/L2QDX.png
[2]: https://i.stack.imgur.com/T9Fz5.png

Answer 1

It seems you are using wrong divider symbols, "_" instead of "-" in your query. Try unique_identifier = '126-89948-2576'

Answer 2

Because the underscore _ is a wildcard like the percent %, except that it only looks for one character.
SQL pattern matching enables you to use "_" to match any single character and "%" to match an arbitrary number of characters (including zero characters).
you can use
SELECT * FROM open_invoices WHERE unique_identifier like '%126\_89948\_2576%'