问题
While editing some queries to add alternatives for columns without values, I accidentally wrote something like this (here is the simplyfied version):
SELECT id, (SELECT name) FROM t
To my surprise, MySQL didn't throw any error, but completed the query giving my expected results (the name column values).
I tried to find any documentation about it, but with no success.
Is this SQL standard or a MySQL specialty?
Can I be sure that the result of this syntax is really the column value from the same (outer) table? The extended version would be like this:
SELECT id, (SELECT name FROM t AS t1 where t1.id=t2.id) FROM t AS t2
but the EXPLAIN reports No tables used in the Extra column for the former version, which I think is very nice.
Here's a simple fiddle on SqlFiddle (it keeps timing out for me, I hope you have better luck).
Clarification: I know about subqueries, but I always wrote subqueries (correlated or not) that implied a table to select from, hence causing an additional step in the execution plan; my question is about this syntax and the result it gives, that in MySQL seems to return the expected value without any.
回答1:
This is the default behavior for the SQL language and it is defined on the SQL ANSI 2011 over ISO/IEC 9075-1:2011(en) documentation. Unfortunately it is not open. This behavior is described on the section 4.11 SQL-Statements.
This behavior happens because the databases process the select comand without the from clause, therefore if it encounters:
select id, (select name) from some
It will try to find that name field as a column of the outer queries to process.
Fortunately I remember that some while ago I've answered someone here and find a valid available link to an SQL ANSI document that is online in FULL but it is for the SQL ANSI 99 and the section may not be the same one as the new document. I think, did not check, that it is around the section 4.30. Take a look. And I really recommend the reading (I did that back in the day).
Database Language SQL - ISO/IEC 9075-2:1999 (E)
回答2:
What you within your first query is a correlated subquery which simply returns the name column from the table t. no actual subquery needs to run here (which is what your EXPLAIN is telling you).
In a SQL database query, a correlated subquery (also known as a synchronized subquery) is a subquery (a query nested inside another query) that uses values from the outer query.
https://en.wikipedia.org/wiki/Correlated_subquery
SELECT id, (SELECT name) FROM t
is the same as
SELECT id, (SELECT t.name) FROM t
Your 2nd query
SELECT id, (SELECT name FROM t AS t1 where t1.id=t2.id) FROM t AS t2
Also contains correlated subquery but this one is actually running a query on table t to find records where t1.id = t2.id.
回答3:
It's not standard. In oracle,
select 1, (select 2)
from dual
Throws error, ORA-00923: FROM keyword not found where expected
How can you be sure of your results? Get a better understanding of what the query is supposed to acheive before you write it. Even the exetended version in the question does not make any sense.
来源：https://stackoverflow.com/questions/34340013/sql-syntax-select-without-from-clause-as-subquery-in-select-subselect