select *
from orders
where extract(year from order_date)='2018'select order_id,ifnull(shipper_id,'not found')
from orders

• //如果第一个列名为null，就返回第二个列名的值，如果第二个列名的值也是null，返回字符串，可以增加的第3个，等等

• select concat(first_name,' ',last_name) as customer,coalesce(phone,'Unknown') as phone
  from customers

• 有时候需要测试条件，并根据条件的成立与否，返回不同值，调用 if 函数，输入一个测试用的表达式，如果表达式判定为true，函数就会返回第一个值，否则返回第二个值

• select order_id,order_date,if (year(order_date)='2019','now','past') as status
  from orders

• 要有好几个表达式判断怎么办？使用case运算符，case when + 表达式 then 'something' …… end

• select order_id,casewhen year(order_date)='2019' then 'now'when year(order_date)='2018' then 'last year'when year(order_date)<'2018' then 'pass'else 'future'end as statue
  from orders;select concat(first_name,' ',last_name) as customer,points,casewhen points>=3000 then 'Gold'when points>=2000 then 'Silver'else 'Bronze'end as category
  from customers
  order by points desc