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  • java写sql语句吗[2021-01-29 10:43:11]简介:php去除nbsp的方法:首先创建一个PHP代码示例文件;然后通过“preg_replace("/(\s|\&nbsp\;| |\xc2\xa0)/", " ", strip_tags($val));”方法去除所有nbsp即可。推荐...

    用java写sql语句吗

    [2021-01-29 10:43:11]  简介:

    879101.html

    php去除nbsp的方法:首先创建一个PHP代码示例文件;然后通过“preg_replace("/(\s|\&nbsp\;| |\xc2\xa0)/", " ", strip_tags($val));”方法去除所有nbsp即可。推荐:《PHP视频教

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    yii2打印sql语句的办法:可以在controller里面执行语句,代码为【$model->find()->createCommand()->getRawSql();】,在网页里点击底部的log就可以看到sql语句。yii2打印sql语句

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    建站服务器

    删除织梦所有待审核稿件sql语句怎么写?这个问题可能是我们日常学习或工作经常见到的。希望通过这个问题能让你收获颇深。下面

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    mysql写循环语句的方法:1、使用while循环,代码为【WHILE i<5 DO】;3、使用repeat循环;代码为【REPEAT INSERT INTO test VALUES(i+11,'test','20')】。【相关学

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    SQL中select语句的语法结构是:“SELECT select_list [ INTO..] FROM table_name [ WHERE...] [ GROUP BY...][ HAVING...][ ORDER BY...];”。SELECT语句由一系列灵活的子句

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    MySQL插入SQL语句后在phpmyadmin中注释乱码的解决办法:首先查询sql语句,然后在代码中添加语句【set names gbk】改变编码;最后重新打开显示正确即可。MySQL插入SQL语句后在phpm

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    Java中return语句有两个作用,分别是:(推荐教程:java入门程序)1、返回方法指定类型的值(这个值总是确定的);2、结束方法的执行(仅仅一个return语句)。return语句用在非void返回值类型的

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    java 学习步骤是:1、进入Java语言基础的学习阶段;2、掌握面向对象语言的共性;3、熟练掌握JDK核心API编程技术;4、熟练的掌握SQL语句;5、熟练掌握JDBC API;6、理解迭代的意义;7、掌

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    下面由Laravel教程栏目给大家介绍Laravel中轻松容易的输出完整的SQL语句的方法,希望对需要的朋友有所帮助!

    larav

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    下面由Laravel教程栏目给大家介绍使用Orator将你的SQL转换为Laravel Query语句的方法,希望对需要的朋友有所帮助!介绍你可以可以使用 Orator(Maurice Calhoun 的在线工具)轻松的

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    我们先来看下面试题:面试题:如何判断SQL查询操作是不是慢sql、如何优化(阿里面试题)面试题:MySQL慢查询开启,语句分析(阿里面试题)(学习视频分享:java教学视频)一、开启mysql慢查询

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    中国网科技7月24日讯 今日,工信部发布今年第三批侵害用户权益行为的APP通报。通报称,截至目前,尚有58款APP未完成整改。对于出现问题的APP,应在7月30日前完成整改落

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    sql语句中的drop关键字的功能是:从数据库中删除表。drop语句将删除表的结构,以及被依赖的约束、触发器和索引。在删除之后,依赖于该表的存储过程、函数将保留,但是将变为invalid

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    打印输出语句有:换行打印“System.out.println();”,不换行打印“System.out.print();”,字节输出“System.out.write();”,按格式输出“System.out.printf();”。Java中的输出

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    java调用mysql数据的方法:首先定义一个过程,获取users表总记录数,将10设置到变量count中;然后修改mysql语句的结果符;接着将结果覆给变量a;最后显示变量a的值。【相关学习推荐:php

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  • 10 Common Mistakes Java Developers Make when WritingSQLJava developers mix object-oriented thinking with imperative thinking, depending on their levels of:Skill (anyone can code imperatively)Dogma (s....

    10 Common Mistakes Java Developers Make when Writing SQL

    Java developers mix object-oriented thinking with imperative thinking, depending on their levels of:

    Skill (anyone can code imperatively)Dogma (some use the “Pattern-Pattern”, i.e. the pattern of applying patterns everywhere and giving them names)Mood (true OO is more clumsy to write than imperative code. At first)

    But when Java developers write SQL, everything changes. SQL is a declarative language that has nothing to do with either object-oriented or imperative thinking. It is very easy to express a query in SQL. It is not so easy to express it optimally or correctly. Not only do developers need to re-think their programming paradigm, they also need to think in terms of set theory.

    Here are common mistakes that a Java developer makes when writing SQL through JDBC or jOOQ (in no particular order). For 10 More Common Mistakes, see this article here.

    Here are common mistakes that a Java developer makes when writing SQL (in no particular order):

    1. Forgetting about NULL

    Misunderstanding NULL is probably the biggest mistake a Java developer can make when writing SQL. This is also (but not exclusively) due to the fact that NULL is also called UNKNOWN. If it were only called UNKNOWN, it would be easier to understand. Another reason is that JDBC maps SQL NULL to Java null when fetching data or when binding variables. This may lead to thinking that NULL = NULL (SQL) would behave the same way as null == null (Java)

    One of the crazier examples of misunderstanding NULL is when NULL predicates are used with row value expressions.

    Another, subtle problem appears when misunderstanding the meaning ofNULL in NOT IN anti-joins.

    The Cure:

    Train yourself. There’s nothing but explicitly thinking about NULL, every time you write SQL:

    Is this predicate correct with respect to NULL?Does NULL affect the result of this function?

    2. Processing data in Java memory

    Few Java developers know SQL very well. The occasional JOIN, the odd UNION, fine. But window functions? Grouping sets? A lot of Java developers load SQL data into memory, transform the data into some appropriate collection type, execute nasty maths on that collection with verbose loop structures (at least, before Java 8’s Collection improvements).

    But some SQL databases support advanced (and SQL standard!) OLAP features that tend to perform a lot better and are much easier to write. A (non-standard) example is Oracle’s awesome MODEL clause. Just let the database do the processing and fetch only the results into Java memory. Because after all some very smart guys have optimised these expensive products. So in fact, by moving OLAP to the database, you gain two things:

    Simplicity. It’s probably easier to write correctly in SQL than in JavaPerformance. The database will probably be faster than your algorithm. And more importantly, you don’t have to transmit millions of records over the wire.

    The Cure:

    Every time you implement a data-centric algorithm in Java, ask yourself: Is there a way to let the database perform that work for me?

    3. Using UNION instead of UNION ALL

    It’s a shame that UNION ALL needs an extra keyword compared to UNION. It would be much better if the SQL standard had been defined to support:

    UNION (allowing duplicates)UNION DISTINCT (removing duplicates)

    Not only is the removal of duplicates rarely needed (or sometimes even wrong), it is also quite slow for large result sets with many columns, as the two subselects need to be ordered, and each tuple needs to be compared with its subsequent tuple.

    Note that even if the SQL standard specifies INTERSECT ALL and EXCEPT ALL, hardly any database implements these less useful set operations.

    The Cure:

    Every time you write a UNION, think if you actually wanted to write UNION ALL.

    4. Using JDBC Pagination to paginate large results

    Most databases support some way of paginating ordered results through LIMIT .. OFFSET, TOP .. START AT, OFFSET .. FETCH clauses. In the absence of support for these clauses, there is still the possibility forROWNUM (Oracle) or ROW_NUMBER() OVER() filtering (DB2, SQL Server 2008 and less), which is much faster than pagination in memory. This is specifically true for large offsets!

    The Cure:

    Just use those clauses, or a tool (such as jOOQ) that can simulate those clauses for you.

    5. Joining data in Java memory

    From early days of SQL, some developers still have an uneasy feeling when expressing JOINs in their SQL. There is an inherent fear of JOIN being slow. This can be true if a cost-based optimiser chooses to perform a nested loop, possibly loading complete tables into database memory, before creating a joined table source. But that happens rarely. With appropriate predicates, constraints and indexes, MERGE JOIN and HASH JOIN operations are extremely fast. It’s all about the correct metadata (I cannot cite Tom Kyte often enough for this). Nonetheless, there are probably still quite a few Java developers who will load two tables from separate queries into maps and join them in Java memory in one way or another.

    The Cure:

    If you’re selecting from various tables in various steps, think again to see if you cannot express your query in a single statement.

    6. Using DISTINCT or UNION to remove duplicates from an accidental cartesian product

    With heavy joining, one can loose track of all the relations that are playing a role in a SQL statement. Specifically, if multi-column foreign key relationships are involved, it is possible to forget to add the relevant predicates in JOIN .. ON clauses. This might result in duplicate records, but maybe only in exceptional cases. Some developers may then choose to use DISTINCT to remove those duplicates again. This is wrong in three ways:

    It (may) solve the symptoms but not the problem. It may as well not solve the symptoms in edge-cases.It is slow for large result sets with many columns. DISTINCT performs an ORDER BY operation to remove duplicates.It is slow for large cartesian products, which will still load lots of data into memory

    The Cure:

    As a rule of thumb, when you get unwanted duplicates, always review your JOIN predicates. There’s probably a subtle cartesian product in there somewhere.

    7. Not using the MERGE statement

    This isn’t really a mistake, but probably some lack of knowledge or some fear towards the powerful MERGE statement. Some databases know other forms of UPSERT statements, e.g. MySQL’s ON DUPLICATE KEY UPDATE clause. But MERGE is really so powerful, most importantly in databases that heavily extend the SQL standard, such as SQL Server.

    The Cure:

    If you’re UPSERTING by chaining INSERT and UPDATE or by chaining SELECT .. FOR UPDATE and then INSERT or UPDATE, think again. Apart from risking race conditions, you might be able to express a simpler MERGE statement.

    8. Using aggregate functions instead of window functions

    Before the introduction of window functions, the only means to aggregate data in SQL was by using a GROUP BY clause along with aggregate functions in the projection. This works well in many cases, and if aggregation data needed to be enriched with regular data, the grouped query can be pushed down into a joined subquery.

    But SQL:2003 defined window functions, which are implemented by many popular database vendors. Window functions can aggregate data on result sets that are not grouped. In fact, each window function supports its own, independent PARTITION BY clause, which is an awesome tool for reporting.

    Using window functions will:

    Lead to more readable SQL (less dedicated GROUP BY clauses in subqueries)Improve performance, as a RDBMS is likely to optimise window functions more easily

    The Cure:

    When you write a GROUP BY clause in a subquery, think again if this cannot be done with a window function.

    9. Using in-memory sorting for sort indirections

    The SQL ORDER BY clause supports many types of expressions, including CASE statements, which can be very useful for sort indirections. You should probably never sort data in Java memory because you think that

    SQL sorting is too slowSQL sorting cannot do it

    The Cure:

    If you sort any SQL data in memory, think again if you cannot push sorting into your database. This goes along well with pushing pagination into the database.

    10. Inserting lots of records one by one

    JDBC knows batching, and you should use it. Do not INSERT thousands of records one by one, re-creating a new PreparedStatement every time. If all of your records go to the same table, create a batch INSERT statement with a single SQL statement and multiple bind value sets. Depending on your database and database configuration, you may need to commit after a certain amount of inserted records, in order to keep the UNDO log slim.

    The Cure:

    Always batch-insert large sets of data.

    Some interesting books

    Some very interesting books on similar topics are

    SQL Antipatterns by Bill KarwinSQL Performance Explained by Markus Winand

    from:http://blog.jooq.org/2013/07/30/10-common-mistakes-java-developers-make-when-writing-sql/

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  • 在VBA使用ADO可以高效的查询数据,但是写SQL查询语句是个有些令人头痛的事情,其实在ADO中用到的SQL语句的语法并不是很复杂,很多时候是由于即要思考SQL的语法,又要用VBA的字符串连接命令,最终搞得一团糟。...

    在VBA使用ADO可以高效的查询数据,但是写SQL查询语句是个有些令人头痛的事情,其实在ADO中用到的SQL语句的语法并不是很复杂,很多时候是由于即要思考SQL的语法,又要用VBA的字符串连接命令,最终搞得一团糟。

    其实,这个创建SQL语句的工作可以交给VBA来完成,当然其前提是——你已经想清楚了要如何构建你的SQL。

    1. 定义参数

    为了和VBA中数组元素编号一致,称为“参数0”到“参数4”,共5个,分别对应SQL语句的不同部分。代码中给出了“聚合SQL”和“非聚合带排序”两个例子。

    2. 使用变量

    参数数组也可以用变量,比如需要查询当前年份的记录,参数3初始赋值为”[年]=’?'”,然后使用下面代码进行替换

    aPara(2) = VBA.Replace(aPara(2), REPLACE_CHAR, Year(Date))

    3. SQL语句模板

    将SQL模板定义为”SELECT ? FROM ? WHERE ? ? ?”,共有5个问号,对应5个参数。

    4. 可选SQL关键字

    由于GROUP BY和ORDER BY是可选部分,因此如果不需要相关部分的话,请将相应的参数置空,参见“非聚合带排序”。

    5. 动态创建SQL字符串

    准备工作已经完成,循环替换就可以创建SQL字符串,Replace方法的第5个参数需要指定为1,以实现逐个替换。

    sSQL = VBA.Replace(sSQL, REPLACE_CHAR, aPara(i), 1, 1, vbTextCompare)

    6. 完整代码

    Sub CreateSQL()

    Dim sSQL, aPara

    Const REPLACE_CHAR = "?"

    '==============================================================

    ' aPara = Array("Fields", _ 参数0: 字段

    ' "Table", _ 参数1: 数据表

    ' "Where", _ 参数2: 条件

    ' "Group By", _ 参数3: 分组

    ' "Order By") 参数4: 排序

    '--------------------------------------------------------------

    ' 聚合SQL

    ' aPara = Array("[班级],[科目],SUM([成绩])", _

    ' "[成绩表$A:K]", _

    ' "[年]='2017'", _

    ' "[班级]", _

    ' "[班级],[科目]")

    '--------------------------------------------------------------

    ' 非聚合带排序

    aPara = Array("[班级],[科目],[姓名],[成绩]", _

    "[成绩表$A:K]", _

    "[年]='2017'", _

    "", _

    "[班级],[科目]")

    '--------------------------------------------------------------

    ' 使用变量替换参数

    aPara(2) = VBA.Replace(aPara(2), REPLACE_CHAR, Year(Date))

    '==============================================================

    sSQL = "SELECT ? FROM ? WHERE ? ? ?"

    For i = 0 To 4

    If i > 2 And Len(aPara(i)) > 0 Then

    aPara(i) = IIf(i = 3, "GROUP BY ", "ORDER BY ") & aPara(i)

    End If

    sSQL = VBA.Replace(sSQL, REPLACE_CHAR, aPara(i), 1, 1, vbTextCompare)

    'Debug.Print sSQL

    Next

    Debug.Print sSQL

    End Sub

    7. SQL查询字符串

    ' 聚合SQL

    SELECT [班级],[科目],SUM([成绩]) FROM [成绩表$A:K] WHERE [年]='2017' GROUP BY [班级] ORDER BY [班级],[科目]

    ' 非聚合带排序

    SELECT [班级],[科目],[姓名],[成绩] FROM [成绩表$A:K] WHERE [年]='2018' ORDER BY [班级],[科目]

    每次只需要稍加修改参数数组就可以创建出SQL语句,感觉不错吧!大家也可以把这个Sub过程改造成Function函数,更便于在代码中调用。

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  • 数据库字段不能为中文,否则java里面写SQL写不出来  select * from 岁末合约_结果; 解决办法==》用as将中文列名 as成对应实体类中的属性名 select user_id as userid, bill_id as billid, 客户姓名 as cusname ,...
    数据库字段不能为中文,否则java里面写SQL写不出来
     
    <span style="font-size:14px;">    select * from 岁末合约_结果;
    </span>

    解决办法==》用as将中文列名 as成对应实体类中的属性名
    <pre name="code" class="sql"><span style="font-size:14px;">select user_id as userid, bill_id as billid, 客户姓名 as cusname , 外呼时间 as outcalltime,
          业务名称 as ywname, 外呼人员 as outcallman, 数据来源 as datasource, 营销成功项目 as yxsucesspro,
          预约时间 as yytime, 余额 as leftmoney, 办理合约名称 as blhyname, 办理时间 as bltime, 办理操作人 as blman,
          办理组织 as blorg, 撤销合约名称 as cxhyname, 撤销时间 as cxtime, 撤销操作人 as cxman, 撤销组织 as cxorg, 
          终止合约名称 as zzhyname, 终止时间 as zzhytime, 终止操作人 as zzhyman, 终止组织 as zzhyorg,批量调整金额 as pltzmoney,
          pos券充值金额 as posczmoney, 新增是否冲突 as isaddct, 更新时间 as updatetime 
          from 岁末合约_结果  a</span>
    
    


    这对于select查询操作时可以的,其他操作没有办法

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  • 用spark实现查数据库对象的方法,话不多说直接撸代码: SparkSession spark = ... .appName("JavaWordCount").master("local") .getOrCreate(); //前提map里面已经有相应的参数了 String url = map.get("...
  • 发现csdn上的连接池都是配设xml的,就手写了一份数据库连接池(java),连接sqlserver,里面一共两个java代码,Conn类包含了Connection和标志位,myconnection包含了数据库连接池的使用:获取连接,增加连接,释放...
  • 记录备忘,直接上步骤: 1,工程里面spring配置文件,配置两个数据源,...2,框架里面设置两个 jdbcTemplate,分别用作读和 3、创建本地发布,发布类型为事务发布,具体过程略, 4、创建本地订阅,具体过程略,...
  • 这边的方法是通过 select xmlelement("",column_name) from table_name 取到一个xml格式的数据,然后在java中转成String再到本地文件中(因为本人技术不高,不知道其他办法,所以就只能多绕一点路来实现目的)。...
  • Java 编程(SQL 优化)

    2020-02-08 23:30:31
    在传统的系统应用程序中我们通常都会和数据库建立连接进行数据的读写操作,...所以本篇 Chat 将主要从 SQL 语句的优化给出一些建议以及如何使用 SQL 语句里面的关键字等才能使 SQL 的执行效率相对提升,希望给研发...
  • javaSQL查询语句有like '%?%'怎么

    万次阅读 多人点赞 2018-04-10 22:50:03
    但是我们想要在java里面写一条sql语句能和这条语句相同。一开始,我在java里面用了这一条语句 当我运行结果时,我发现报了一个java.lang.StackOverflowError异常,我也去网上收了一些答案, 一开始我也不知道是...
  • 今天的工作遇到了关于JAVA方面的转义的知识...今天在一条查询的SQL语句的时候遇到了查询条件中含有_的情况,通过上网查找资料使用的ESCAPE关键字 ,这个关键字的主要作用是匹配sql语句里面的下划线。 select gid,...
  • 某天,在群里看见有个人 问 sqlService里面有2000万条数据 需要转储到orcale中。什么方式比较好,后来一琢磨非多线程莫属啊~;我这里没有那么多数据,所以我就模拟了下 一次存入10万条数据,看有多快~;上代码。在...
  • 【实例简介】这是本人花了半个月出来的一个基于java编程和SqlServer数据库技术结合的Swing项目,是一个超市管理系统,里面包含着超市管理的基本业务流程,(员工管理,部门管理,商品管理,销售管理,)还包含数据库...
  • 你真拿你这点分当宝贝了...大体了下自己看吧。你就是1000分也没人愿意帮你做的,这里的1000分1RMB都不值。public void write(){try{//创建一个可写入的excel文件对象WritableWorkbook workbook = Workbook.createW...
  • import java.sql.*;public class GetConn {public Connection conn = null; // 创建Connection对象// 获取数据库连接方法public Connection getConnection() {try {Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); ...
  • 任何系统里,日志和一定的监控是相当重要的,在一个软件整个生命周期里维护永远是大头同时是痛苦的,而日志和监控就是为后期维护提供了良好的基础和手段,在java工程里面大多使用log4j来记录系统日志,这个技术几乎...
  • 这个是MyEclipse连接SQL Server JAVA源代码代码,里面都是好的方法,直接可以调用
  • 一、场景描述平时在用字符串拼接sql语句的时候,有没有遇到过下面的情况:没错,就是单双引号问题。...二、问题分析平时写sql的时候,遇到条件为字符串的值,我们会习惯性的加上单引号''其实,在sql语...
  • 里面有用到sql语句,同时函数参数包括(num,ID),即购买数量和商品ID   第一次sql语句如下: String sql = "update items set number=number-num where id='ID';"; (这是错误的) (number为表...
  • sql server2008连接eclipse,用java写的某书店入库销售系统(数据库综合实训),里面含有sqljdbc文件,不过的比较粗糙,见谅,大家一起学习
  • 项目上都是利用hibernate的原始态调用sql,由于涉及的表比较多,所以在写sql查询出来的速度很慢,我们在优化的时候,把一些表的结果集放在临时表上了,这样提升了一些速度。但是我现在在java的源代码上拿不到结果集 ...
  • Java sql查询

    千次阅读 2017-11-09 14:36:28
    对于数据库查询,也就是jdbc,我常常是完一条SQL语句然后new一个prepareStatment对象然后执行该语句,查询的条件通常在sql语句里面死了。 但如果要关联网页的话,也就是获取网页中的值来查询的话,就要动态的...
  • 众所周知,Sql里面居然没有数组类型,让大家每次在存储过程或者触发器中处理数组的时候都很麻烦。我也遇到这样的情况,所以就特意了两个自定函数,以 方便自己今后在Sql中处理数组。入行不久,懂得不多,贴出来...
  • 这边的方法是通过 select xmlelement("",column_name) from table_name 取到一个xml格式的数据,然后在java中转成String再到本地文件中(因为本人技术不高,不知道其他办法,所以就只能多绕一点路来实现目的)。...
  •  某天,在群里看见有个人 问 sqlService里面有2000万条数据 需要转储到orcale中。什么方式比较好,后来一琢磨非多线程莫属啊~; 我这里没有那么多数据,所以我就模拟了下 一次存入10万条数据,看有多快~;上代码。...
  • 这是本人花了半个月出来的一个基于java编程和SqlServer数据库技术结合的Swing项目,是一个超市管理系统,里面包含着超市管理的基本业务流程,(员工管理,部门管理,商品管理,销售管理,)还包含数据库设计,数据...
  • 了一个替换,不知道里面“”该怎么处理?谢谢帮忙 ``` public static String SqlReplaceStr(String str) { str = str.replace("'", """); str = str.replace("%", "?"); //这里该怎么才能保留%号的原意,而...
  • 主要方法是Java写好控制文件拼接好sqlldr命令字符串,通过Runtime类在终端命令行执行。 但是在执行的时候报错了,java.io.IOException: Cannot run program "sqlldr": error=2, A file or directory in the path ...
  • 1,新建java项目(或者web项目,这里以java为例),以user表为例子 ...3,这里我将数据库的连接代码封装在DBConnectionUtil.java里面 package com.beiruan.util; import java.io.IOException; imp

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