MySQL 8.0

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SELECT VERSION();

VERSION()

8.0.13

DROP TABLE IF EXISTS `t`;
CREATE TABLE IF NOT EXISTS `t` (
  `f` VARCHAR(255)
);
INSERT INTO `t`
VALUES ('ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(128) Mac=AEAD');
SELECT SUBSTRING_INDEX(`f`, SPACE(1), 1)
FROM `t`;

SUBSTRING_INDEX(`f`, SPACE(1), 1)

ECDHE-RSA-AES128-GCM-SHA256