【SQL】神奇的DUAL表 mysql文档中对于dual表的解释: You are allowed to specify DUAL as a dummy table name in situations where no tables are referenced: 你可以在没有表的情况下指定一个虚拟的表名 mysql> SELECT 1 + 1 FROM DUAL; -> 2 DUAL is purely for the convenience of people who require ...
The DUAL is special one row, one column table present by default in all Oracle databases. The owner of DUAL is SYS (SYS owns the data dictionary, therefore DUAL is part of the data dictionary.) but DUAL can be accessed by every user. The table has a single VARCHAR2(1) column called ...
——拉罗什富科 前两天看到项目中有这样一句SQL SELECT (SELECT username FROM `user_2018` WHERE i...
1.Whatisthe dual table, whatisits purpose. dualisjust a convienence table. You don't need to use it, you can use anything youwant. The advantagetodualisthe optimizer understands dualisa special one row, one column table --whenyou use itinqueries, it uses this knowledgewhendeveloping the p...
oracle sql 替换 in ,dual性能优化 create table bakInfo( id int, pname varchar2(10), remark varchar2(10), impdate varchar2(10), upstate varchar(10)); create table Info( id int, pname varchar2(10), remark varchar2(10), impdate varchar2(10),...
Dual表在测试、调试和执行一些简单计算时非常有用,而不需要创建实际的表。 希望本文对你理解SQL Server中的Dual表有所帮助! 参考资料: [SQL Server DUAL Table]( [How to emulate the Oracle DUAL table in MySQL]( [How to emulate the Oracle DUAL table in PostgreSQL](...
In Oracle, theSELECTstatement must have aFROMclause. However, some queries don’t require any table for example: SELECTUPPER('This is a string')FROMwhat_table;Code language:SQL (Structured Query Language)(sql) In this case, you might think about creating a table and use it in theFROMclaus...
Name SQL-05: Hide reliance on the dual table. Synopsis This is a special case of [SQL-04: Put single-row fetches inside functions; never hard-code a query in your block.] … - Selection from Oracle PL/SQL Best Practices [Book]
dual是一个表,有一个名为DUMMY,类型为VARCHAR2(1)。DUMMY这个词在英文中的含义为“something designed to resemble and serve as a substitute for the real...
Alternatively, you can select a constant, pseudocolumn, or expression from any table, but the value will be returned as many times as there are rows in the table. Refer to "About SQL Functions" for many examples of selecting a constant value from DUAL. Note: Beginning with Oracle Data...