In MySQL,

CONCAT('foo', 'bar', NULL) = NULL

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Read more Show archive.org snapshot

the NULL always wins in MySQL.

If you would rather treat NULL as an empty string, use CONCAT_WS Show archive.org snapshot (concatenation with separator) instead:

CONCAT_WS('', 'foo', 'bar', NULL) = 'foobar'

PostgreSQL

In PostgreSQL the NULL is not viral in CONCAT:

CONCAT('foo', 'bar', NULL) = 'foobar'