数据库查询的 PHP 和 MySQL 问题

Question

I have a MySQL table with userId, movieId and date. I'm having issues with my query. I would like to query and list out different date tagged with a movieId under each userId.

userId movieId  date
  1     A     1:00PM
  1     A     3:00PM
  2     A     2:00PM
  2     C     8:00PM
  2     C     9:00PM

I want something like this

UserId --Movie(Time)
  1    A     1:00PM, 3:00PM
  2    A     2:00PM
  2    C     8:00PM,9:00PM

Answer 1

use group_concat function

select UserId ,movieId, group_concat(date separator ',') 
from your_table group by UserId ,movieId 

Answer 2

EDIT: I don't have enough reputation to comment on the accepted answer above but it WILL break on later versions of MySQL and I don't think it's what the poster actually wants - it will not produce the output they said they wanted.

You need to use GROUP BY to collapse several rows into one. And then you want to use an aggregating function to join the data in those rows. An aggregate function is one that takes several rows as input and does something with them to return a single value. In this case you want group_concat.

HOWEVER, you need to tell MySQL what to do with EVERY SINGLE ELEMENT you select from the results if you use GROUP BY. This is because in ANSI SQL a statement must be deterministic. Ie return the same output every time for the same input. Older versions of MySQL would 'guess' values for you but newer ones (5.7.5 and after) by default wont. Also, it's good practice to explicitly tell MySQL how to handle each value anyway. So you want something like this:

SELECT userId, ANY_VALUE(movieId), GROUP_CONCAT(date SEPARATOR ", ") 
FROM tableName GROUP BY UserId;

The above will give you each user id and the list of dates. But the movie code will be randomly selected.

OR you want something like this:

SELECT userId, GROUP_CONCAT(movieId SEPARATOR ", "), GROUP_CONCAT(date SEPARATOR ", ")
FROM tableName GROUP BY UserId;

This will give you the same as before but it will list all the movie codes instead of randomly selecting any value.

OR you want to group by userId AND movieID so that you can have a row for each user and movie combination. Then you would change the GROUP BY clause at the end, like this:

SELECT userId, movieId, GROUP_CONCAT(date SEPARATOR ", ")
FROM tableName GROUP BY UserId, movieId;

I've included three different options because I think you may actually want the third one even though you asked for the first.

MySQL version matters. If you are before MySQL 5.7 then the AnyValue(…) function doesn't exist and you should simply remove it and put "movieId" by itself. But if you ware after MySQL 5.7 and also if you want to do things the right way, you should include it.

Also, two suggestions. Firstly, don't use letters for the movieID unless you have a good reason to. Use integers like you have for the userId and put any information about the movies in a separate table that the movieID is a key for. Secondly, add a column called "id" to your table and make it the Primary Key. This makes many things a lot easier. You can set the id field to be AUTO_INCREMENT - that's the normal thing to do.

Hope this helps.