如果年龄为空，则保留当前值
我不明白你的意思。列的值是多少？

db.query(`UPDATE ${usersTable}
SET age = CASE WHEN age IS NULL THEN age ELSE $2 END,
    password = CASE WHEN password IS NULL THEN age ELSE $3 END,
    sex = CASE WHEN sex IS NULL THEN age ELSE $4 END,
    tel = CASE WHEN tel IS NULL THEN age ELSE $5 END,
    avatar = CASE WHEN avatar IS NULL THEN age ELSE $6 END
WHERE email = $1 RETURNING *`, 
            [email, age, password, sex, tel, avatar])

如果你指的是变量age，那么我想你应该试着用编程语言中的'null'字符串值来代替null值。