There is an approach to designing systems with an idea of never deleting or mutating data called Event Sourcing. Basically, the idea is to store events (or facts) that change the system state, instead of snapshots of the state. The history of events can be replayed later on to produce a certain purpose-specific projection of what the state at any point in time looked like. Multiple projections built for different purposes can coexist in the system. More information can found on the following web sites:

 - http://martinfowler.com/eaaDev/EventSourcing.html
 - http://codebetter.com/gregyoung/2010/02/20/why-use-event-sourcing/

It&#39;s in line with what you are describing, but rather than being just a database model, Event Sourcing and Command Query Responsibility Segregation (CQRS) prescribe a special way of designing the whole system including the database and business logic layers.

There are a few frameworks that follow this approach, such as:

 - http://www.axonframework.org/
 - http://qi4j.org/
 - http://en.jdon.com/

While this does not directly answer your question, it may provide a different perspective on the problem.

[Irmin][1] is a distributed database that follows the same design principles as Git.

  [1]: https://github.com/mirage/irmin

The `\K` escape sequence resets the beginning of the match to the current position in
the token list (this only affects what is reported as the full match).
 
What environments/languages/versions support `\K` (keep) in its regular expression engines and what libraries are needed (if any) to use this feature within patterns?

Support of \K in regex

What am I doing wrong here? I&#39;m simply retrieving results from a table and then adding them to an array. Everything works as expected until I check for an empty result...

This gets the match, adds it to my array and echoes the result as expected:

    $today = date(&#39;Y-m-d&#39;, strtotime(&#39;now&#39;));

    $sth = $db-&gt;prepare(&quot;SELECT id_email FROM db WHERE hardcopy = &#39;1&#39; AND hardcopy_date &lt;= :today AND hardcopy_sent = &#39;0&#39; ORDER BY id_email ASC&quot;);

    $sth-&gt;bindParam(&#39;:today&#39;, $today, PDO::PARAM_STR);

    if(!$sth-&gt;execute()) {
        $db = null;
        exit();
    }

    while ($row = $sth-&gt;fetch(PDO::FETCH_ASSOC)) {
        $this-&gt;id_email[] = $row[&#39;id_email&#39;];
        echo $row[&#39;id_email&#39;];
    }

    $db = null;
    return true;

When I try to check for an empty result, my code returns &#39;empty&#39;, but no longer yields the matching result:

    $today = date(&#39;Y-m-d&#39;, strtotime(&#39;now&#39;));

    $sth = $db-&gt;prepare(&quot;SELECT id_email FROM db WHERE hardcopy = &#39;1&#39; AND hardcopy_date &lt;= :today AND hardcopy_sent = &#39;0&#39; ORDER BY id_email ASC&quot;);

    $sth-&gt;bindParam(&#39;:today&#39;,$today, PDO::PARAM_STR);

    if(!$sth-&gt;execute()) {
        $db = null;
        exit();
    }

    if ($sth-&gt;fetchColumn()) {
        echo &#39;not empty&#39;;
        while ($row = $sth-&gt;fetch(PDO::FETCH_ASSOC)) {
            $this-&gt;id_email[] = $row[&#39;id_email&#39;];
            echo $row[&#39;id_email&#39;];
        }
        $db = null;
        return true;
    }
    echo &#39;empty&#39;;
    $db = null;
    return false;


Checking for an empty result (PHP, PDO, and MySQL)

I&#39;m very much inspired by the approach to data management [advocated by Rich Hickey](http://www.infoq.com/author/Rich-Hickey), and implemented in [Datomic](http://www.datomic.com/), where the data is never mutated in-place, all the versions are _always_ preserved and query-able, and the time is a first-class concept.

Of course, there are specialized databases matching that description, like Git, or any other source control system. The question is if there are any (more or less) general-purpose DBMS-es of relational, graph, hierarchical, document or any other flavor that can be effectively used in, say, an eCommerce Web application. Or is Datomic the only choice then?

General-purpose databases that never delete or update data in-place

I'm very much inspired by the approach to data management <a href="http://www.infoq.com/author/Rich-Hickey" target="_blank" rel="noopener noreferrer">advocated by Rich Hickey</a>, and implemented in <a href="http://www.datomic.com/" target="_blank" rel="noopener noreferrer">Datomic</a>, where the data is never mutated in-place, all the versions are always preserved and query-able, and the time is a first-class concept.
Of course, there are specialized databases matching that description, like Git, or any other source control system. The question is if there are any (more or less) general-purpose DBMS-es of relational, graph, hierarchical, document or any other flavor that can be effectively used in, say, an eCommerce Web application. Or is Datomic the only choice then?

Why do I need Transaction in Hibernate for read-only operations?  

Does the following transaction put a lock in the DB? 

Example code to fetch from DB:

    Transaction tx = HibernateUtil.getCurrentSession().beginTransaction(); // why begin transaction?
    //readonly operation here
    
    tx.commit() // why tx.commit? I don&#39;t want to write anything

Can I use `session.close()`  instead of `tx.commit()`?

Why do I need Transaction in Hibernate for read-only operations?

I&#39;m trying to achieve a &quot;One to one&quot; relationship in a MySQL database. For example, let&#39;s say I have a Users table and an Accounts table. I want to be sure that a User can have only one Account. And that there can be only one Account per User.

I found two solutions for this but don&#39;t know what to use, and are there any other options.

### First solution:

    DROP DATABASE IF EXISTS test;
    CREATE DATABASE test CHARSET = utf8 COLLATE = utf8_general_ci;
    USE test;
    
    CREATE TABLE users(
    	id INT NOT NULL AUTO_INCREMENT,
    	user_name VARCHAR(45) NOT NULL,
    	PRIMARY KEY(id)
    ) ENGINE = InnoDB DEFAULT CHARSET = utf8;
    
    CREATE TABLE accounts(
    	id INT NOT NULL AUTO_INCREMENT,
    	account_name VARCHAR(45) NOT NULL,
    	user_id INT UNIQUE,
    	PRIMARY KEY(id),
    	FOREIGN KEY(user_id) REFERENCES users(id)
    ) ENGINE = InnoDB DEFAULT CHARSET = utf8;

In this example, I define the foreign key in accounts pointing to the primary key in users.
And then I make foreign key UNIQUE, so there can&#39;t be two identical users in accounts.
To join tables I would use this query:

    SELECT * FROM users JOIN accounts ON users.id = accounts.user_id;

### Second solution:

    DROP DATABASE IF EXISTS test;
    CREATE DATABASE test CHARSET = utf8 COLLATE = utf8_general_ci;
    USE test;
    
    CREATE TABLE users(
    	id INT NOT NULL AUTO_INCREMENT,
    	user_name VARCHAR(45) NOT NULL,
    	PRIMARY KEY(id)
    ) ENGINE = InnoDB DEFAULT CHARSET = utf8;
    
    CREATE TABLE accounts(
    	id INT NOT NULL AUTO_INCREMENT,
    	account_name VARCHAR(45) NOT NULL,
    	PRIMARY KEY(id),
    	FOREIGN KEY(id) REFERENCES users(id)
    ) ENGINE = InnoDB DEFAULT CHARSET = utf8;

In this example, I create a foreign key that points from the primary key to a primary key in another table. Since Primary Keys are UNIQUE by default, this makes this relation One to One.
To join tables I can use this:

    SELECT * FROM users JOIN accounts ON users.id = accounts.id;

Now the questions:

 - What is the best way to create One to One relation in MySQL?
 - Are there any other solutions other than these two?

I&#39;m using MySQL Workbench, and when I design One To One relation in EER diagram and let MySQL Workbench produce SQL code, I get One to Many relation :S That&#39;s what&#39;s confusing me :S

And if I import any of these solutions into MySQL Workbench EER diagram, it recognizes relations as One to Many :S That&#39;s also confusing.

So, what would be the best way to define One to One relation in MySQL DDL. And what options are there to achieve this?


MySQL - One To One Relationship?

I want to disable cascade deletes for a link table with entity framework code-first.  For example, if many users have many roles, and I try to delete a role, I want that delete to be blocked *unless* there are no users currently associated with that role.  I already remove the cascade delete convention in my `OnModelCreating`:

	protected override void OnModelCreating(DbModelBuilder modelBuilder) {
		...
		modelBuilder.Conventions.Remove&lt;OneToManyCascadeDeleteConvention&gt;();

And then I set up the user-role link table:

	modelBuilder.Entity&lt;User&gt;()
		.HasMany(usr =&gt; usr.Roles)
		.WithMany(role =&gt; role.Users)
		.Map(m =&gt; {
			m.ToTable(&quot;UsersRoles&quot;);
			m.MapLeftKey(&quot;UserId&quot;);
			m.MapRightKey(&quot;RoleId&quot;);
		});

Yet when EF creates the database, it creates a delete cascade for the foreign key relationships, eg.

	ALTER TABLE [dbo].[UsersRoles]  WITH CHECK ADD  CONSTRAINT [FK_dbo.UsersRoles_dbo.User_UserId] FOREIGN KEY([UserId])
	REFERENCES [dbo].[User] ([UserId])
	ON DELETE CASCADE
	GO
	
	ALTER TABLE [dbo].[UsersRoles]  WITH CHECK ADD  CONSTRAINT [FK_dbo.UsersRoles_dbo.Role_RoleId] FOREIGN KEY([RoleId])
	REFERENCES [dbo].[Role] ([RoleId])
	ON DELETE CASCADE
	GO

How can I stop EF generating this delete cascade?

How to disable cascade delete for link tables in EF code-first?

I have to insert about 2 million rows from a text file.

And with inserting I have to create some master tables. 

What is the best and fast way to insert such a large set of data into SQL Server?



Insert 2 million rows into SQL Server quickly

I&#39;m in a corporate environment (running Debian Linux) and didn&#39;t install it myself. I access the databases using Navicat or phpPgAdmin (if that helps). I also don&#39;t have shell access to the server running the database.

Which version of PostgreSQL am I running?

What&#39;s the difference between:

    class Person(name: String, age: Int) {
      def say = &quot;My name is &quot; + name + &quot;, age &quot; + age
    }

and

    class Person(val name: String, val age: Int) { 
      def say = &quot;My name is &quot; + name + &quot;, age &quot; + age
    }

Can I declare parameters as `var`s, and change their values later? For instance,

    class Person(var name: String, var age: Int) {

      age = happyBirthday(5)

      def happyBirthday(n: Int) {
        println(&quot;happy &quot; + n + &quot; birthday&quot;)
        n
      }
    }

scala class constructor parameters

Must immutable objects have all properties be `final`?

According to me not. But I don&#39;t know, whether I am right.


Must all properties of an immutable object be final?

    import copy

    a = &quot;deepak&quot;
    b = 1, 2, 3, 4
    c = [1, 2, 3, 4]
    d = {1: 10, 2: 20, 3: 30}

    a1 = copy.copy(a)
    b1 = copy.copy(b)
    c1 = copy.copy(c)
    d1 = copy.copy(d)


    print(&quot;immutable - id(a)==id(a1)&quot;, id(a) == id(a1))
    print(&quot;immutable - id(b)==id(b1)&quot;, id(b) == id(b1))
    print(&quot;mutable - id(c)==id(c1)&quot;, id(c) == id(c1))
    print(&quot;mutable - id(d)==id(d1)&quot;, id(d) == id(d1))

I get the following results:

    immutable - id(a)==id(a1) True
    immutable - id(b)==id(b1) True
    mutable - id(c)==id(c1) False
    mutable - id(d)==id(d1) False

If I perform deepcopy:

    a1 = copy.deepcopy(a)
    b1 = copy.deepcopy(b)
    c1 = copy.deepcopy(c)
    d1 = copy.deepcopy(d)

results are the same:

    immutable - id(a)==id(a1) True
    immutable - id(b)==id(b1) True
    mutable - id(c)==id(c1) False
    mutable - id(d)==id(d1) False

If I work on assignment operations:

    a1 = a
    b1 = b
    c1 = c
    d1 = d

then results are:

    immutable - id(a)==id(a1) True
    immutable - id(b)==id(b1) True
    mutable - id(c)==id(c1) True
    mutable - id(d)==id(d1) True

Can somebody explain what exactly makes a difference between the copies? Is it something related to mutable &amp; immutable objects? If so, can you please explain it to me?

What is the difference between shallow copy, deepcopy and normal assignment operation?

My goal is to make a Java object immutable. I have a class `Student`. I coded it in the following way to achieve immutability:

    public final class Student {

    private String name;
    private String age;

    public Student(String name, String age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public String getAge() {
        return age;
    }

    }

My question is, what is the best way to achieve immutability for the `Student` class? 

Make immutable Java object

We all know that `String` is immutable in Java, but check the following code:

    String s1 = &quot;Hello World&quot;;  
    String s2 = &quot;Hello World&quot;;  
    String s3 = s1.substring(6);  
    System.out.println(s1); // Hello World  
    System.out.println(s2); // Hello World  
    System.out.println(s3); // World  
    
    Field field = String.class.getDeclaredField(&quot;value&quot;);  
    field.setAccessible(true);  
    char[] value = (char[])field.get(s1);  
    value[6] = &#39;J&#39;;  
    value[7] = &#39;a&#39;;  
    value[8] = &#39;v&#39;;  
    value[9] = &#39;a&#39;;  
    value[10] = &#39;!&#39;;  
         
    System.out.println(s1); // Hello Java!  
    System.out.println(s2); // Hello Java!  
    System.out.println(s3); // World  

Why does this program operate like this? And why is the value of `s1` and `s2` changed, but not `s3`?


Is a Java string really immutable?

I&#39;m intrigued in the database service Datomic, but I&#39;m not sure if it fits the needs of the projects I work on. When is Datomic a good choice, and when should it be avoided?

Content Type	Original Author	Original Content on Stackoverflow
Question	Ivan Krechetov	View Question on Stackoverflow
Solution 1 - Database	Anton Beloglazov	View Answer on Stackoverflow
Solution 2 - Database	Dzmitry Lahoda	View Answer on Stackoverflow

General-purpose databases that never delete or update data in-place

Database Problem Overview

Database Solutions

Solution 1 - Database

Solution 2 - Database

Checking for an empty result (PHP, PDO, and MySQL)

Support of \K in regex

Attributions