问题
I noticed when doing a query with multiple JOINs that my query didn't work unless I gave one of the table names an alias.
Here's a simple example to explain the point:
This doesn't work:
SELECT subject
from items
join purchases on items.folder_id=purchases.item_id
join purchases on items.date=purchases.purchase_date
group by folder_id
This does:
SELECT subject
from items
join purchases on items.folder_id=purchases.item_id
join purchases as p on items.date=p.purchase_date
group by folder_id
Can someone explain this?
回答1:
You are using the same table Purchases twice in the query. You need to differentiate them by giving a different name.
You need to give an alias:
When the same table name is referenced multiple times
Imagine two people having the exact same John Doe. If you call John, both will respond to your call. You can't give the same name to two people and assume that they will know who you are calling. Similarly, when you give the same resultset named exactly the same, SQL cannot identify which one to take values from. You need to give different names to distinguish the result sets so SQL engine doesn't get confused.
Script 1: t1 and t2 are the alias names here
SELECT t1.col2
FROM table1 t1
INNER JOIN table1 t2
ON t1.col1 = t2.col1
When there is a derived table/sub query output
If a person doesn't have a name, you call them and since you can't call that person, they won't respond to you. Similarly, when you generate a derived table output or sub query output, it is something unknown to the SQL engine and it won't what to call. So, you need to give a name to the derived output so that SQL engine can appropriately deal with that derived output.
Script 2: t1 is the alias name here.
SELECT col1
FROM
(
SELECT col1
FROM table1
) t1
回答2:
The only time it is REQUIRED to provide an alias is when you reference the table multiple times and when you have derived outputs (sub-queries acting as tables) (thanks for catching that out Siva). This is so that you can get rid of ambiguities between which table reference to use in the rest of your query.
To elaborate further, in your example:
SELECT subject
from items
join purchases on items.folder_id=purchases.item_id
join purchases on items.date=purchases.purchase_date
group by folder_id
My assumption is that you feel that each join and its corresponding on will use the correlating table, however you can use whichever table reference you want. So, what happens is that when you say on items.date=purchases.purchase_date, the SQL engine gets confused as to whether you mean the first purchases table, or the second one.
By adding the alias, you now get rid of the ambiguities by being more explicit. The SQL engine can now say with 100% certainty which version of purchases that you want to use. If it has to guess between two equal choices, then it will always throw an error asking for you to be more explicit.
回答3:
It is required to give them a name when the same table is used twice in a query. In your case, the query wouldn't know what table to choose purchases.purchase_date from.
回答4:
In this case it's simply that you've specified purchases twice and the SQL engine needs to be able to refer to each dataset in the join in a unique way, hence the alias is needed.
As a side point, do you really need to join into purchases twice? Would this not work:
SELECT
subject
from
items
join purchases
on items.folder_id=purchases.item_id
and items.date=purchases.purchase_date
group by folder_id
回答5:
The alias are necessary to disambiguate the table from which to get a column.
So, if the column's name is unique in the list of all possible columns available in the tables in the from list, then you can use the coulmn name directly.
If the column's name is repeated in several of the tables available in the from list, then the DB server has no way to guess which is the right table to get the column.
In your sample query all the columns names are duplicated because you're getting "two instances" of the same table (purchases), so the server needs to know from which of the instance to take the column. SO you must specify it.
In fact, I'd recommend you to always use an alias, unless there's a single table. This way you'll avoid lots of problems, and make the query much more clear to understand.
回答6:
You can't use the same table name in the same query UNLESS it is aliased as something else to prevent an ambiguous join condition. That's why its not allowed. I should note, it's also better to use always qualify table.field or alias.field so other developers behind you don't have to guess which columns are coming from which tables.
When writing a query, YOU know what you are working with, but how about the person behind you in development. If someone is not used to what columns come from what table, it can be ambiguous to follow, especially out here at S/O. By always qualifying by using the table reference and field, or alias reference and field, its much easier to follow.
select
SomeField,
AnotherField
from
OneOfMyTables
Join SecondTable
on SomeID = SecondID
compare that to
select
T1.SomeField,
T2.AnotherField
from
OneOfMyTables T1
JOIN SecondTable T2
on T1.SomeID = T2.SecondID
In these two scenarios, which would you prefer reading... Notice, I've simplified the query using shorter aliases "T1" and "T2", but they could be anything, even an acronym or abbreviated alias of the table names... "oomt" (one of my tables) and "st" (second table). Or, as something super long as has been in other posts...
Select * from ContractPurchaseOffice_AgencyLookupTable
vs
Select * from ContractPurchaseOffice_AgencyLookupTable AgencyLkup
If you had to keep qualifying joins, or field columns, which would you prefer looking at.
Hope this clarifies your question.
来源：https://stackoverflow.com/questions/10433183/when-is-it-required-to-give-a-table-name-an-alias-in-sql