MariaDB 10.2

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CREATE TABLE test (id INT, myDate DATE)
SELECT 1 AS id, STR_TO_DATE('n.b.', '%e.%c.%Y') AS myDate


id
myDate


1
null


INSERT INTO test SELECT 1 AS id, STR_TO_DATE('n.b.', '%e.%c.%Y') AS myDate
Incorrect datetime value: 'n.b.' for function str_to_date

INSERT INTO
  test
WITH
  de_reference AS
(
  SELECT 1 AS id, STR_TO_DATE('n.b.', '%e.%c.%Y') AS myDate
)
SELECT id, myDate FROM de_reference
Incorrect datetime value: 'n.b.' for function str_to_date

INSERT INTO test VALUES (1, STR_TO_DATE('n.b.', '%e.%c.%Y'))
Incorrect datetime value: 'n.b.' for function str_to_date

INSERT INTO
  test
WITH
  source AS
(
  SELECT 1 AS id, 'n.b.' AS string
),
  de_reference AS
(
  SELECT id, STR_TO_DATE(string, '%e.%c.%Y') AS myDate FROM source
)
SELECT id, myDate FROM de_reference
Incorrect datetime value: 'n.b.' for function str_to_date

WITH
  source AS
(
            SELECT 1 AS id, 'n.b.' AS string
  UNION ALL SELECT 2 AS id, '1.1.2019' AS string
  UNION ALL SELECT 3 AS id, '33.33.2019' AS string

)
SELECT
  id,
  CASE WHEN
    string REGEXP '[0-9]{1,2}\.[0-9]{1,2}\.[0-9]{4}'
  THEN
    STR_TO_DATE(string, '%e.%c.%Y')
  ELSE
    NULL
  END
     AS myDate
FROM
  source


id
myDate


1
null

2
2019-01-01

3
null


INSERT INTO
  test
WITH
  source AS
(
            SELECT 1 AS id, 'n.b.' AS string
  UNION ALL SELECT 2 AS id, '1.1.2019' AS string
  UNION ALL SELECT 3 AS id, '33.33.2019' AS string

)
SELECT
  id,
  CASE WHEN
    string REGEXP '[0-9]{1,2}\.[0-9]{1,2}\.[0-9]{4}'
  THEN
    STR_TO_DATE(string, '%e.%c.%Y')
  ELSE
    NULL
  END
     AS myDate
FROM
  source
Incorrect datetime value: '33.33.2019' for function str_to_date

SELECT * FROM test
id	myDate
1	null
2	2019-01-01
3	null