mysql 中 = 运算符是什么意思

MySQL中的:=运算符是什么意思？

sql
SELECT name, age, age + 10 AS new_age
FROM students;

sql
SELECT name, age, 
    CASE 
        WHEN age < 18 THEN '未成年'
        ELSE '成年'
    END AS age_group,
    @is_adult := CASE 
                    WHEN age >= 18 THEN '是'
                    ELSE '否'
                 END AS is_adult
FROM students;