mysql几个常用的判空函数：isnull, ifnull, nullif, coalesce

如果expr为null，则返回1，否则返回0

根isnull类似，expr1不为null的情况下，返回expr1，返回expr2

nullif和ifnull完全不同，它表示如果两个表达式相同，则返回null，否则返回expr1的值

个人感觉应该叫null if equals

发音是core or less，英文的意思是联合/合并，在mysql中的用途是返回第一个不是null的值

只有两个参数的情况下，相当于ifnull

php 　中　mysqli　是个类，这个类的函数（方法）有：

mysqli::$affected_rows — Gets the number of affected rows in a previous MySQL operation

mysqli::autocommit — 打开或关闭本次数据库连接的自动命令提交事务模式

mysqli::begin_transaction — Starts a transaction

mysqli::change_user — Changes the user of the specified database connection

mysqli::character_set_name — 返回当前数据库连接的默认字符编码

mysqli::$client_info — Get MySQL client info

mysqli::$client_version — Returns the MySQL client version as a string

mysqli::close — 关闭先前打开的数据库连接

mysqli::commit — 提交一个事务

mysqli::$connect_errno — Returns the error code from last connect call

mysqli::$connect_error — Returns a string description of the last connect error

mysqli::__construct — Open a new connection to the MySQL server

mysqli::debug — Performs debugging operations

mysqli::dump_debug_info — 将调试信息输出到日志

mysqli::errno — 返回最近函数调用的错误代码

mysqli::$error_list — Returns a list of errors from the last command executed

mysqli::$error — Returns a string description of the last error

mysqli::$field_count — Returns the number of columns for the most recent query

mysqli::get_charset — Returns a character set object

mysqli::get_client_info — Get MySQL client info

mysqli_get_client_stats — Returns client per-process statistics

mysqli_get_client_version — 作为一个整数返回MySQL客户端的版本

mysqli::get_connection_stats — Returns statistics about the client connection

mysqli::$host_info — 返回一个表述使用的连接类型的字符串

mysqli::$protocol_version — 返回MySQL使用的协议版本号

mysqli::$server_info — 返回MySQL服务器的版本号

mysqli::$server_version — 作为一个整数返回MySQL服务器的版本

mysqli::get_warnings — Get result of SHOW WARNINGS

mysqli::$info — Retrieves information about the most recently executed query

mysqli::init — Initializes MySQLi and returns a resource for use with mysqli_real_connect()

mysqli::$insert_id — Returns the auto generated id used in the last query

mysqli::kill — Asks the server to kill a MySQL thread

mysqli::more_results — Check if there are any more query results from a multi query

mysqli::multi_query — Performs a query on the database

mysqli::next_result — Prepare next result from multi_query

mysqli::options — Set options

mysqli::ping — Pings a server connection, or tries to reconnect if the connection has gone down

mysqli::poll — Poll connections

mysqli::prepare — Prepare an SQL statement for execution

mysqli::query — 对数据库执行一次查询

mysqli::real_connect — 建立一个 MySQL 服务器连接

mysqli::real_escape_string — Escapes special characters in a string for use in an SQL statement, taking into account the current charset of the connection

mysqli::real_query — 执行一个mysql查询

mysqli::reap_async_query — Get result from async query

mysqli::refresh — Refreshes

mysqli::release_savepoint — Removes the named savepoint from the set of savepoints of the current transaction

mysqli::rollback — 回退当前事务

mysqli::rpl_query_type — Returns RPL query type

mysqli::savepoint — Set a named transaction savepoint

mysqli::select_db — 选择用于数据库查询的默认数据库

mysqli::send_query — 发送请求并返回结果

mysqli::set_charset — 设置默认字符编码

mysqli::set_local_infile_default — Unsets user defined handler for load local infile command

mysqli::set_local_infile_handler — Set callback function for LOAD DATA LOCAL INFILE command

mysqli::$sqlstate — Returns the SQLSTATE error from previous MySQL operation

mysqli::ssl_set — Used for establishing secure connections using SSL

mysqli::stat — Gets the current system status

mysqli::stmt_init — 初始化一条语句并返回一个用于mysqli_stmt_prepare(调用)的对象

mysqli::store_result — Transfers a result set from the last query

mysqli::$thread_id — Returns the thread ID for the current connection

mysqli::thread_safe — 返回是否是线程安全的

mysqli::use_result — Initiate a result set retrieval

mysqli::$warning_count — Returns the number of warnings from the last query for the given link

以上函数清单直接来自　www.php.net　网站。你可以进入该网站参看。

分别在 MySQL5.7.25-log 和 8.0.16 环境中实现类似Oracle的分析函数(8.0版本中已支持，直接使用即可)。

一、创建测试数据

二、row_number() over()

三、rank() over()

四、dense_rank() over()

五、lag() over()

六、lead() over()

七、待补充

例1：不分组，全部数据添加序列号，类Oracle 的rownum伪列

例2：先按roomid分组，再按照deviceid,counter排序，类Oracle 的row_number() OVER(PARTITION BY ORDER BY )

例1：不分组，全部数据按 roomid 排序，再添加序号，类Oracle 的rank() OVER(ORDER BY)

例2：先按roomid分组，再按deviceid排序，类Oracle 的rank() OVER(PARTITION BY ORDER BY)

例1：不分组，全部数据按roomid排序，再添加序号，类Oracle 的dense_rank() OVER(ORDER BY)

例2：先按roomid分组，再按deviceid排序，类Oracle 的dense_rank() OVER(PARTITION BY ORDER BY)

例1：不分组，全部数据按roomid,deviceid升序排序，类Oracle 的lag() OVER(ORDER BY)

例2：先按roomid分组，再按roomid,deviceid排序，类Oracle 的lag() OVER(PARTITION BY ORDER BY)

例1：不分组，全部数据按roomid,deviceid,counter升序排序，类Oracle 的lead() OVER(ORDER BY)

例2：先按roomid分组，再按deviceid,counter排序，类Oracle 的lead() OVER(PARTITION BY ORDER BY)

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