问题描述
在SQL Server中可以执行SELECT，而无需参考表；类似:
选择 1.2 +3, 'my dummy string'由于Oracle 不允许没有FROM 的SELECT，因此我使用双表进行此类操作；类似:
从 DUAL 中选择 1,2+3, 'my dummy string'有没有更好的方法来做这种类型的查询?使用双表是个好习惯吗?
不，在Oracle中没有SELECT没有FROM.
使用 dual 表是一个好习惯.
dual 是一个内存表.如果您不从中选择 DUMMY，它将使用不需要 I/O 的特殊访问路径 (FAST DUAL).>
曾几何时，dual 有两条记录(因此得名)，旨在作为虚拟记录集来复制被连接的记录.
现在它只有一条记录，但你仍然可以用它生成任意数量的行:
SELECT 级别从双联系方式等级 <= 100MySQL 还支持 dual(以及 fromless 语法).
in SQL Server is possible to execute a SELECT, without reference to a table; something like:
Select 1.2 +3, 'my dummy string'
As Oracle does not allow a SELECT without a FROM, I use the dual table for this type of operation; something like:
Select 1,2+3, 'my dummy string' FROM DUAL
There is a better way of doing this type of query? it is good practice to use the dual table?
No, in Oracle there is no SELECT without FROM.
Using the dual table is a good practice.
dual is an in-memory table. If you don't select DUMMY from it, it uses a special access path (FAST DUAL) which requires no I/O.
Once upon a time, dual had two records (hence the name) and was intended to serve as a dummy recordset to duplicate records being joined with.
Now it has but one record, but you can still generate an arbitrary number of rows with it:
SELECT level
FROM dual
CONNECT BY
level <= 100
MySQL also supports dual (as well as the fromless syntax).
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