Oracle使用两种光标:显式光标和隐式光标。不管语句返回多少条纪录,PL/SQL为使用的每一条UPDATE、DELETE和INSERT等SQL命令隐式的
怎样统计PLSQL语言中删除语句执行之后一共删除了多少条纪录。
Oracle使用两种光标:显式光标和隐式光标。不管语句返回多少条纪录,PL/SQL为使用的每一条UPDATE、DELETE和INSERT等SQL命令隐式的声明一个光标。(要管理SQL语句的处理,必须隐式的给它定义一个光标。)
在PL/SQL中,,当执行DML语句的时候,会自动打开一个隐式游标(游标就类似JDBC中的ResultSet),而相关要被操作的数据都是会先被放入隐式游标。隐式游标由ORACLE帮我们自动管理,如果想要手动进行一些操作的话,我们可以使用隐式游标的相关属性。如:
SQL%FOUND 成功获取记录返回true,否则返回false
SQL%NOTFOUND 成功获取记录返回true,否则返回false
SQL%ROWCOUNT 返回从游标中获取的记录条数
SQL%ISOPEN 总是返回false
利用隐式游标属性SQL%ROWCOUNT可以达到计算删除了多少条纪录。
可以获取的DML sql包括:INSERT /UPDATE/DELETE /MERGER
具体使用看下面的例子把
create or replace procedure test_02 is
CNT INT;
begin
DELETE FROM TEST_01 ;
cnt:=sql%rowcount;
commit;
end;
create or replace procedure test_03 is
CNT INT;
n_1 int;
n_2 int;
n_3 int;
begin
merge into test_04
using (select * from test_09 )
merger_subquery
on(test_04.object_id = merger_subquery.object_id)
when not matched then
insert (object_id,OBJECT_NAME)
values (merger_subquery.object_id,merger_subquery.object_name)
when matched then
update set object_name = merger_subquery.object_name;
cnt:=sql%rowcount;
n_1:=cnt;
commit;
end;

InnoDB uses redologs and undologs to ensure data consistency and reliability. 1.redologs record data page modification to ensure crash recovery and transaction persistence. 2.undologs records the original data value and supports transaction rollback and MVCC.

Key metrics for EXPLAIN commands include type, key, rows, and Extra. 1) The type reflects the access type of the query. The higher the value, the higher the efficiency, such as const is better than ALL. 2) The key displays the index used, and NULL indicates no index. 3) rows estimates the number of scanned rows, affecting query performance. 4) Extra provides additional information, such as Usingfilesort prompts that it needs to be optimized.

Usingtemporary indicates that the need to create temporary tables in MySQL queries, which are commonly found in ORDERBY using DISTINCT, GROUPBY, or non-indexed columns. You can avoid the occurrence of indexes and rewrite queries and improve query performance. Specifically, when Usingtemporary appears in EXPLAIN output, it means that MySQL needs to create temporary tables to handle queries. This usually occurs when: 1) deduplication or grouping when using DISTINCT or GROUPBY; 2) sort when ORDERBY contains non-index columns; 3) use complex subquery or join operations. Optimization methods include: 1) ORDERBY and GROUPB

MySQL/InnoDB supports four transaction isolation levels: ReadUncommitted, ReadCommitted, RepeatableRead and Serializable. 1.ReadUncommitted allows reading of uncommitted data, which may cause dirty reading. 2. ReadCommitted avoids dirty reading, but non-repeatable reading may occur. 3.RepeatableRead is the default level, avoiding dirty reading and non-repeatable reading, but phantom reading may occur. 4. Serializable avoids all concurrency problems but reduces concurrency. Choosing the appropriate isolation level requires balancing data consistency and performance requirements.

MySQL is suitable for web applications and content management systems and is popular for its open source, high performance and ease of use. 1) Compared with PostgreSQL, MySQL performs better in simple queries and high concurrent read operations. 2) Compared with Oracle, MySQL is more popular among small and medium-sized enterprises because of its open source and low cost. 3) Compared with Microsoft SQL Server, MySQL is more suitable for cross-platform applications. 4) Unlike MongoDB, MySQL is more suitable for structured data and transaction processing.

MySQL index cardinality has a significant impact on query performance: 1. High cardinality index can more effectively narrow the data range and improve query efficiency; 2. Low cardinality index may lead to full table scanning and reduce query performance; 3. In joint index, high cardinality sequences should be placed in front to optimize query.

The MySQL learning path includes basic knowledge, core concepts, usage examples, and optimization techniques. 1) Understand basic concepts such as tables, rows, columns, and SQL queries. 2) Learn the definition, working principles and advantages of MySQL. 3) Master basic CRUD operations and advanced usage, such as indexes and stored procedures. 4) Familiar with common error debugging and performance optimization suggestions, such as rational use of indexes and optimization queries. Through these steps, you will have a full grasp of the use and optimization of MySQL.

MySQL's real-world applications include basic database design and complex query optimization. 1) Basic usage: used to store and manage user data, such as inserting, querying, updating and deleting user information. 2) Advanced usage: Handle complex business logic, such as order and inventory management of e-commerce platforms. 3) Performance optimization: Improve performance by rationally using indexes, partition tables and query caches.


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