expr1条件，条件为true，则值是expr2 ，false，值就是expr3

SELECT 
o.id,u.account,catagory.`name`,orderTime,detail.amount,periodtime,if(direction='0','看涨','看跌') directionName
FROM t_order_detail detail
LEFT JOIN t_order o ON o.id = detail.orderId
LEFT JOIN t_catagory catagory ON catagory.id = o.catId
LEFT JOIN t_user u ON u.id = o.userId
2. CASE WHEN
CASE 列名
    WHEN 条件 THEN 结果 
    ELSE 其他结果
    END 别名
SELECT 
CASE detail.`status` WHEN '0' THEN '未开仓'
WHEN '1' THEN '已开仓'
WHEN '2' THEN '已平仓'
ELSE '取消订单' END
status,
CASE o.type WHEN '0' THEN '单期'
WHEN '1' THEN '多期'
ELSE '策略' END 
typeName
FROM t_order_detail detail
LEFT JOIN t_order o ON o.id = detail.orderId
3. IFNULL
  表达式：IFNULL( expr1 , expr2)
  在 expr1 的值不为 NULL的情况下都返回 expr1，否则返回 expr2
select IFNULL(detailcode,'无') detailcode from ...