Return Null Values by String Literal, PostgreSQL

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DROP TABLE IF EXISTS mydata;

CREATE TABLE mydata (
    mytype VARCHAR(20),
    mynum NUMERIC(6,4),
    mychar VARCHAR(3),
    mydate TIMESTAMP
);
  
INSERT INTO mydata VALUES ('mysql',45.2707,'LlB', '2018-06-18'),
                          (NULL,91.5861,'d7I', '2015-05-10'),
                          ('sql server',77.9822,'4Wn', '2017-01-29'),
                          ('db2',85.5323,'sqz', '2016-01-05'),
                          (NULL, 5.8967,'fS1', '2018-07-24'),
                          ('mysql',54.8832,'1hp', '2015-11-01'),
                          ('sqlite',63.0003,'vxS', '2016-02-26'),
                          ('sql server',68.3834,'wB8', '2015-02-16'),
                          (NULL,30.7427,'CD3', '2016-06-08'),
                          ('sql server',98.9217,'SXz', '2015-09-29'),
                          ('db2',29.7813,'h1D', '2017-04-30'),
                          ('postgres',97.0466,'fjD', '2018-02-08'),
                          ('mysql',30.0664,'Snh', '2015-07-06'),
                          (NULL,27.6271,'TSs', '2016-10-21'),
                          ('db2',66.1275,'XT8', '2018-11-17'),
                          ('sqlite',83.3873,'Tea', '2018-03-20'),
                          ('mysql',32.3265,'yU1', '2018-08-01'),
                          (NULL, 1.8472,'EH5', '2017-10-27'),
                          ('mysql',17.9318,'BmM', '2018-09-21'),
                          ('oracle',41.1535,'14M', '2017-04-30'),
                          ('sql server', 4.6209,'5RJ', '2017-01-26'),
                          ('db2',81.6313,'RPQ', '2018-11-22');

SELECT * FROM mydata
WHERE COALESCE(mytype, 'default') = 'default';


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