mysql：'r'是什么意思？

Question

I've been going through some code and repeatedly see the letter 'r' in queries, and I've never seen it before. I'm no mysql guru and cannot find references on the web that really make sense in this case.

Sample query:

SELECT * FROM database.table r
WHERE column = 'whatever'
AND otherColumn = 'whenever'
ORDER BY id, name

Answer 1

That actually means that the table is being aliased from its long form to the letter/symbol 'r'.

It's a red herring in your case because 'r' is not used anywhere in the query and it doesn't need to be. Your query is not a good example of using aliases because there is only one table involved. If you join multiple tables then aliasing becomes handy (although not required) to specify which table's column you're referencing in your various query's clauses.

You can simply remove the 'r' and run your query.

SELECT * FROM database.table r
WHERE column = 'whatever'
AND otherColumn = 'whenever'
ORDER BY id, name

Or use it outright like: (though it's redundant here)

SELECT * FROM database.table r
WHERE r.column = 'whatever'
AND r.otherColumn = 'whenever'
ORDER BY r.id, r.name

BTW, SQL code like this is the reason I tend to use the keyword AS to highlight the fact that I am aliasing. So the FROM clause would look like this: FROM database.table AS r

As far as what that unused alias is doing there is a good question. My guess it that it's been getting cut, copied and pasted from some old query which used the alias but no one ever bother to remove it when it became unnecessary.

Answer 2

r is an alias you can give to a table. You can reference table columns later by that alias, for example r.column1

Answer 3

It's a table alias. If you're joining two tables with duplicated column names, you would disambiguate the query by saying some_table.id, the table alias allows you to just type r.id.

Answer 4

It's an alias. It means: We must use r now instead of database.table .

We can use r.column but we are still allowed to use just column if there is no ambiguity.

SELECT * FROM database.table r
WHERE r.column = 'whatever'         --using the alias r
AND otherColumn = 'whenever'        --not using it
ORDER BY id, r.name                 --mixing

Answer 5

Looks like a table alias to me with the AS keyword omitted. In the sample query, the alias is unnecessary since only one table is being referenced and the alias is never used.

The relevant syntax for a table_reference is specified in the MySQL documentation as tbl_name [[AS] alias] [index_hint_list] .