查看所有用户 select * from all_users; select * from dba_users;select * from user_users; 查看SID select * from v$instance; 查看所有表空间 select * from v$tablespace; 查询表空间文件所在路径 select * from dba_data_files; 一:取得系统当前时间
查看所有用户
select * from all_users; select * from dba_users; select * from user_users;
查看SID
select * from v$instance;
查看所有表空间
select * from v$tablespace;查询表空间文件所在路径
select * from dba_data_files;
一:取得系统当前时间
SQLServer select getdate(); Oracle select sysdate from dual;
二:日期比较
SQLServer select * from tab where Order_date > '1990-04-13'; Oracle select * from tab where Order_date > date '1990-04-13';
三:NOt
Not逻辑运算符用来将表达式结果进行"反向"判断,以下将以比较运算符中的不等于(!=)并利用比较运算符中的等于(=)与逻辑运算符Not进行结合
select count(*) from orders where book_name != 'java基础' select count(*) from orders where not book_name = 'java基础'
由上述SQL语法可知,当NOT逻辑运算符与比较运算符合并使用时 是将NOT置于比较运算符的前面
四:order by排序
select book_id,book_name from orders order by 1 desc,2 desc

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Best practices for handling string data types and indexes in MySQL include: 1) Selecting the appropriate string type, such as CHAR for fixed length, VARCHAR for variable length, and TEXT for large text; 2) Be cautious in indexing, avoid over-indexing, and create indexes for common queries; 3) Use prefix indexes and full-text indexes to optimize long string searches; 4) Regularly monitor and optimize indexes to keep indexes small and efficient. Through these methods, we can balance read and write performance and improve database efficiency.


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