我不明白这个order_by子句在做什么以及我是否需要它：
select c.customerid, c.firstname, c.lastname, i.order_date, i.item, i.price from
items_ordered i, customers c
where i.customerid = c.customerid
group by c.customerid, i.item, i.order_date
order by i.order_date desc;
这会产生以下数据：
10330 Shawn Dalton 30-Jun-1999 Pogo stick 28.00
10101 John Gray 30-Jun-1999 Raft 58.00
10410 Mary Ann Howell 30-Jan-2000 Unicycle 192.50
10101 John Gray 30-Dec-1999 Hoola Hoop 14.75
10449 Isabela Moore 29-Feb-2000 Flashlight 4.50
10410 Mary Ann Howell 28-Oct-1999 Sleeping Bag 89.22
10339 Anthony Sanchez 27-Jul-1999 Umbrella 4.50
10449 Isabela Moore 22-Dec-1999 Canoe 280.00
10298 Leroy Brown 19-Sep-1999 Lantern 29.00
10449 Isabela Moore 19-Mar-2000 Canoe paddle 40.00
10413 Donald Davids 19-Jan-2000 Lawnchair 32.00
10330 Shawn Dalton 19-Apr-2000 Shovel 16.75
10439 Conrad Giles 18-Sep-1999 Tent 88.00
10298 Leroy Brown 18-Mar-2000 Pocket Knife 22.38
10299 Elroy Keller 18-Jan-2000 Inflatable Mattress 38.00
10438 Kevin Smith 18-Jan-2000 Tent 79.99
10101 John Gray 18-Aug-1999 Rain Coat 18.30
10449 Isabela Moore 15-Dec-1999 Bicycle 380.50
10439 Conrad Giles 14-Aug-1999 Ski Poles 25.50
10449 Isabela Moore 13-Aug-1999 Unicycle 180.79
10101 John Gray 08-Mar-2000 Sleeping Bag 88.70
10299 Elroy Keller 06-Jul-1999 Parachute 1250.00
10438 Kevin Smith 02-Nov-1999 Pillow 8.50
10101 John Gray 02-Jan-2000 Lantern 16.00
10315 Lisa Jones 02-Feb-2000 Compass 8.00
10449 Isabela Moore 01-Sep-1999 Snow Shoes 45.00
10438 Kevin Smith 01-Nov-1999 Umbrella 6.75
10298 Leroy Brown 01-Jul-1999 Skateboard 33.00
10101 John Gray 01-Jul-1999 Life Vest 125.00
10330 Shawn Dalton 01-Jan-2000 Flashlight 28.00
10298 Leroy Brown 01-Dec-1999 Helmet 22.00
10298 Leroy Brown 01-Apr-2000 Ear Muffs 12.50
如果我完全删除了order_by子句，就像在这个查询中一样：
select c.customerid, c.firstname, c.lastname, i.order_date, i.item, i.price from
items_ordered i, customers c
where i.customerid = c.customerid
group by c.customerid, i.item, i.order_date;
我得到了这些结果：
10101 John Gray 30-Dec-1999 Hoola Hoop 14.75
10101 John Gray 02-Jan-2000 Lantern 16.00
10101 John Gray 01-Jul-1999 Life Vest 125.00
10101 John Gray 30-Jun-1999 Raft 58.00
10101 John Gray 18-Aug-1999 Rain Coat 18.30
10101 John Gray 08-Mar-2000 Sleeping Bag 88.70
10298 Leroy Brown 01-Apr-2000 Ear Muffs 12.50
10298 Leroy Brown 01-Dec-1999 Helmet 22.00
10298 Leroy Brown 19-Sep-1999 Lantern 29.00
10298 Leroy Brown 18-Mar-2000 Pocket Knife 22.38
10298 Leroy Brown 01-Jul-1999 Skateboard 33.00
10299 Elroy Keller 18-Jan-2000 Inflatable Mattress 38.00
10299 Elroy Keller 06-Jul-1999 Parachute 1250.00
10315 Lisa Jones 02-Feb-2000 Compass 8.00
10330 Shawn Dalton 01-Jan-2000 Flashlight 28.00
10330 Shawn Dalton 30-Jun-1999 Pogo stick 28.00
10330 Shawn Dalton 19-Apr-2000 Shovel 16.75
10339 Anthony Sanchez 27-Jul-1999 Umbrella 4.50
10410 Mary Ann Howell 28-Oct-1999 Sleeping Bag 89.22
10410 Mary Ann Howell 30-Jan-2000 Unicycle 192.50
10413 Donald Davids 19-Jan-2000 Lawnchair 32.00
10438 Kevin Smith 02-Nov-1999 Pillow 8.50
10438 Kevin Smith 18-Jan-2000 Tent 79.99
10438 Kevin Smith 01-Nov-1999 Umbrella 6.75
10439 Conrad Giles 14-Aug-1999 Ski Poles 25.50
10439 Conrad Giles 18-Sep-1999 Tent 88.00
10449 Isabela Moore 15-Dec-1999 Bicycle 380.50
10449 Isabela Moore 22-Dec-1999 Canoe 280.00
10449 Isabela Moore 19-Mar-2000 Canoe paddle 40.00
10449 Isabela Moore 29-Feb-2000 Flashlight 4.50
10449 Isabela Moore 01-Sep-1999 Snow Shoes 45.00
10449 Isabela Moore 13-Aug-1999 Unicycle 180.79
我不确定order_by在这里做了什么以及它是否具有预期效果。
答案 0 :(得分：3)
看起来它是在i.ordered_date上订购，但是使用字符串比较而不是日期比较，这就是为什么1999年6月30日在2000年2月29日之前放置。作为字符串“30-Jun-1999”＆gt; “2000年2月28日”，但作为日期，情况恰恰相反。
检查i.ordered_date表中items_ordered的类型 - 它应该是datetime或类似的 - 如果它是varchar，那么你需要将其更改为日期类型，或者将值转换为order-by子句中的日期。 E.g。
order by CAST(i.order_date AS DATE) desc
答案 1 :(得分：0)
您应始终使用正确的DATETIME数据类型来存储日期