I started by googling and found the article *[How to write INSERT if NOT EXISTS queries in standard SQL][1]* which talks about mutex tables.

I have a table with ~14 million records. If I want to add more data in the same format, is there a way to ensure the record I want to insert does not already exist without using a pair of queries (i.e., one query to check and one to insert is the result set is empty)?

Does a `unique` constraint on a field guarantee the `insert` will fail if it&#39;s already there?

It seems that with *merely* a constraint, when I issue the insert via PHP, the script croaks.

  [1]: http://www.xaprb.com/blog/2005/09/25/insert-if-not-exists-queries-in-mysql/


How can I do &#39;insert if not exists&#39; in MySQL?

I have a table:

    table votes (
        id,
    	user,
    	email,
    	address,
    	primary key(id),
    );

Now I want to make the columns **user, email, address** unique (together).

How do I do this in MySql?

Of course the example is just... an example. So please don&#39;t worry about the semantics.

How do I specify unique constraint for multiple columns in MySQL?

I have an email column that I want to be unique. But I also want it to accept null values. Can my database have 2 null emails that way?

Does MySQL ignore null values on unique constraints?

I have a table in PostgreSQL where the schema looks like this:

    CREATE TABLE &quot;foo_table&quot; (
        &quot;id&quot; serial NOT NULL PRIMARY KEY,
        &quot;permalink&quot; varchar(200) NOT NULL,
        &quot;text&quot; varchar(512) NOT NULL,
        &quot;timestamp&quot; timestamp with time zone NOT NULL
    )

Now I want to make the permalink unique across the table by `ALTER`-ing the table.

How do I ALTER a PostgreSQL table and make a column unique?

Can I add a UNIQUE constraint to a PostgreSQL table, after it&#39;s already created?

Trying to create this example table structure in Postgres 9.1:

```pgsql
CREATE TABLE foo (
	name        VARCHAR(256) PRIMARY KEY
);

CREATE TABLE bar (
	pkey 	    SERIAL PRIMARY KEY,
	foo_fk      VARCHAR(256) NOT NULL REFERENCES foo(name),	
	name        VARCHAR(256) NOT NULL, 
	UNIQUE (foo_fk,name)
);

CREATE TABLE baz(	
	pkey 	    SERIAL PRIMARY KEY,
	bar_fk      VARCHAR(256) NOT NULL REFERENCES bar(name),
	name        VARCHAR(256)
);
```
    
Running the above code produces an error, which does not make sense to me:

&gt;     NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index &quot;foo_pkey&quot; for table &quot;foo&quot;
&gt;     NOTICE:  CREATE TABLE will create implicit sequence &quot;bar_pkey_seq&quot; for serial column &quot;bar.pkey&quot;
&gt;     NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index &quot;bar_pkey&quot; for table &quot;bar&quot;
&gt;     NOTICE:  CREATE TABLE / UNIQUE will create implicit index &quot;bar_foo_fk_name_key&quot; for table &quot;bar&quot;
&gt;     NOTICE:  CREATE TABLE will create implicit sequence &quot;baz_pkey_seq&quot; for serial column &quot;baz.pkey&quot;
&gt;     NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index &quot;baz_pkey&quot; for table &quot;baz&quot;
&gt;     ERROR:  there is no unique constraint matching given keys for referenced table &quot;bar&quot;
    
    
    ********** Error **********
    
    ERROR: there is no unique constraint matching given keys for referenced table &quot;bar&quot;
    SQL state: 42830

Can anyone explain why this error arises?

ERROR: there is no unique constraint matching given keys for referenced table &quot;bar&quot;

I have a Java bean. Now, I want to be sure that the field should be unique.

I am using the following code:

    @UniqueConstraint(columnNames={&quot;username&quot;})
    public String username;

But I&#39;m getting some error:

    @UniqueConstraint is dissallowed for this location

What&#39;s the proper way to use unique constraints?

**Note:** I am using play framework.


@UniqueConstraint annotation in Java

I can find syntax &quot;charts&quot; on this on the SQLite website, but no examples and my code is crashing. I have other tables with unique constraints on a single column, but I want to add a constraint to the table on two columns.  This is what I have that is causing an SQLiteException with the message &quot;syntax error&quot;.

    CREATE TABLE name (column defs) 
    UNIQUE (col_name1, col_name2) ON CONFLICT REPLACE

I&#39;m doing this based on the following:

[table-constraint][1]

To be clear, the documentation on the link I provided says that `CONTSTRAINT name` should come before my constraint definition.

Something that may lead to the solution though is that whatever follows my parenthesized column definitions is what the debugger complains about.

If I put 

    ...last_column_name last_col_datatype) CONSTRAINT ...

the error is **near &quot;CONSTRAINT&quot;: syntax error**

If I put 

    ...last_column_name last_col_datatype) UNIQUE ...

the error is **near &quot;UNIQUE&quot;: syntax error**

  [1]: http://www.sqlite.org/syntaxdiagrams.html#table-constraint


SQLite table constraint - unique on multiple columns

I have an already existing table with a field that should be unique but is not. I only know this because an entry was made into the table that had the same value as another, already existing, entry and this caused problems. 

How do I make this field only accept unique values?

MySQL - Make an existing Field Unique

I&#39;d like to add a constraint which enforces uniqueness on a column only in a portion of a table.

    ALTER TABLE stop ADD CONSTRAINT myc UNIQUE (col_a) WHERE (col_b is null);

The `WHERE` part above is wishful thinking.

Any way of doing this? Or should I go back to the relational drawing board?

Postgresql: Conditionally unique constraint

Naming conventions are important, and primary key and foreign key have commonly used and obvious conventions (`PK_Table` and `FK_Table_ReferencedTable`, respectively).  The `IX_Table_Column` naming for indexes is also fairly standard.

What about the UNIQUE constraint?  Is there a commonly accepted naming convention for this constraint?  I&#39;ve seen `UK_TableName_Column`, `UQ_TableName_Column`, and someone recommending `AX_TableName_Column` - I don&#39;t know where that comes from.

I&#39;ve typically used `UQ` but I don&#39;t particularly like it, and I do not enjoy having to defend my choice of using it against a `UK` advocate.

I would simply like to see if there is a consensus on the most prevalent naming, or a good reasoning as to why one makes more sense than the others.



Naming convention for unique constraint

I have SQL Server 2008 R2 and I want to set a unique column.  

There seems to be two ways to do this: &quot;unique index&quot; and &quot;unique constraint&quot;.  They are not much different from what I understand, although unique constraint is recommended by most, because you also get an index automatically.

How do I create a unique constraint?  

    ALTER TABLE Customer ADD CONSTRAINT U_Name UNIQUE(Name)


Is there a way to create a unique constraint through the SQL Server Management Studio?

How can I create a unique constraint on my column (SQL Server 2008 R2)?

**Question**

Is it possible to define a unique constraint on a property using either the fluent syntax or an attribute? If not, what are the workarounds?

I have a user class with a primary key, but I would like to make sure the email address is also unique. Is this possible without editing the database directly?


**Solution (based on Matt&#39;s answer)**

	public class MyContext : DbContext {
		public DbSet&lt;User&gt; Users { get; set; }

		public override int SaveChanges() {
			foreach (var item in ChangeTracker.Entries&lt;IModel&gt;())
				item.Entity.Modified = DateTime.Now;

			return base.SaveChanges();
		}

		public class Initializer : IDatabaseInitializer&lt;MyContext&gt; {
			public void InitializeDatabase(MyContext context) {
				if (context.Database.Exists() &amp;&amp; !context.Database.CompatibleWithModel(false))
					context.Database.Delete();

				if (!context.Database.Exists()) {
					context.Database.Create();
					context.Database.ExecuteSqlCommand(&quot;alter table Users add constraint UniqueUserEmail unique (Email)&quot;);
				}
			}
		}
	}

Unique Constraint in Entity Framework Code First

I have to add a unique constraint to an existing table. This is fine except that the table has millions of rows already, and many of the rows violate the unique constraint I need to add. 

What is the fastest approach to removing the offending rows? I have an SQL statement which finds the duplicates and deletes them, but it is taking forever to run. Is there another way to solve this problem? Maybe backing up the table, then restoring after the constraint is added?

How to delete duplicate entries?

I&#39;m following up in regards to a [question that I asked earlier][1] in which I sought to seek a conversion from a goofy/poorly written mysql query to postgresql. I believe I succeeded with that. Anyways, I&#39;m using data that was manually moved from a mysql database to a postgres database. I&#39;m using a query that looks like so:

      UPDATE krypdos_coderound cru

      set is_correct = case 
          when t.kv_values1 = t.kv_values2 then True 
          else False 
          end

      from 
      
      (select cr.id, 
        array_agg(
        case when kv1.code_round_id = cr.id 
        then kv1.option_id 
        else null end 
        ) as kv_values1,

        array_agg(
        case when kv2.code_round_id = cr_m.id 
        then kv2.option_id 
        else null end 
        ) as kv_values2

        from krypdos_coderound cr
         join krypdos_value kv1 on kv1.code_round_id = cr.id
         join krypdos_coderound cr_m 
           on cr_m.object_id=cr.object_id 
           and cr_m.content_type_id =cr.content_type_id 
         join krypdos_value kv2 on kv2.code_round_id = cr_m.id

       WHERE
         cr.is_master= False
         AND cr_m.is_master= True 
         AND cr.object_id=%s 
         AND cr.content_type_id=%s 

       GROUP BY cr.id  
      ) t

    where t.id = cru.id
        &quot;&quot;&quot; % ( self.object_id, self.content_type.id)
      )

I have reason to believe that this works well. However, this has lead to a new issue. When trying to submit, I get an error from django that states:

    IntegrityError at (some url): 
    duplicate key value violates unique constraint &quot;krypdos_value_pkey&quot;

I&#39;ve looked at several of the responses posted on here and I haven&#39;t quite found the solution to my problem (although the related questions have made for some interesting reading). I see this in my logs, which is interesting because I never explicitly call insert- django must handle it:


       STATEMENT:  INSERT INTO &quot;krypdos_value&quot; (&quot;code_round_id&quot;, &quot;variable_id&quot;, &quot;option_id&quot;, &quot;confidence&quot;, &quot;freetext&quot;)
       VALUES (1105935, 11, 55, NULL, E&#39;&#39;) 
       RETURNING &quot;krypdos_value&quot;.&quot;id&quot;

However, trying to run that results in the duplicate key error. The actual error is thrown in the code below.

     # Delete current coding         CodeRound.objects.filter(object_id=o.id,content_type=object_type,is_master=True).delete()
      code_round = CodeRound(object_id=o.id,content_type=object_type,coded_by=request.user,comments=request.POST.get(&#39;_comments&#39;,None),is_master=True)
      code_round.save()
      for key in request.POST.keys():
        if key[0] != &#39;_&#39; or key != &#39;csrfmiddlewaretoken&#39;:
          options = request.POST.getlist(key)
          for option in options:
            Value(code_round=code_round,variable_id=key,option_id=option,confidence=request.POST.get(&#39;_confidence_&#39;+key, None)).save()  #This is where it dies
      # Resave to set is_correct
      code_round.save()
      o.status = &#39;3&#39; 
      o.save(

I&#39;ve checked the sequences and such and they seem to be in order. At this point I&#39;m not sure what to do- I assume it&#39;s something on django&#39;s end but I&#39;m not sure. Any feedback would be much appreciated!

  [1]: https://stackoverflow.com/questions/11000068/changing-a-weird-mysql-query-to-postgresql-that-uses-group-concat

IntegrityError duplicate key value violates unique constraint - django/postgres

I need to catch violation of `UNIQUE` constraints in a special way by a C# application I am developing. Is it safe to assume that `Error 2627` will always correspond to a violation of this kind, so that I can use 

    if (ThisSqlException.Number == 2627)
    {
        // Handle unique constraint violation.
    }
    else
    {
        // Handle the remaing errors.
    }

?

Unique Key Violation in SQL Server - Is it safe to assume Error 2627?

I have a situation where I want to use the Meta options of `unique_together` to enforce a certain rule, here&#39;s the intermediary model:

&lt;!-- language: python --&gt;

    class UserProfileExtension(models.Model):
        extension = models.ForeignKey(Extension, unique=False)
        userprofile = models.ForeignKey(UserProfile, unique=False)
        user = models.ForeignKey(User, unique=False)  
        
        class Meta:
            unique_together = ((&quot;userprofile&quot;, &quot;extension&quot;),
                               (&quot;user&quot;, &quot;extension&quot;),
                               # How can I enforce UserProfile&#39;s Client 
                               # and Extension to be unique? This obviously
                               # doesn&#39;t work, but is this idea possible without
                               # creating another FK in my intermediary model 
                               (&quot;userprofile__client&quot;, &quot;extension&quot;))



and here&#39;s UserProfile:

&lt;!-- language: python --&gt;

    class UserProfile(models.Model):
        user = models.ForeignKey(User, unique=True)
        client = models.ForeignKey(Client)


Thanks.


Django Unique Together (with foreign keys)

So I know in &lt;a href=&quot;http://www.mysql.com/&quot;&gt;MySQL&lt;/a&gt; it&#39;s possible to insert multiple rows in one query like so:
 
    INSERT INTO table (col1,col2) VALUES (1,2),(3,4),(5,6)

I would like to delete multiple rows in a similar way. I know it&#39;s possible to delete multiple rows based on the exact same conditions for each row, i.e.

    DELETE FROM table WHERE col1=&#39;4&#39; and col2=&#39;5&#39;
or
    
    DELETE FROM table WHERE col1 IN (1,2,3,4,5)

However, what if I wanted to delete multiple rows in one query, with each row having a set of conditions unique to itself? Something like this would be what I am looking for:

    DELETE FROM table WHERE (col1,col2) IN (1,2),(3,4),(5,6)

Does anyone know of a way to do this? Or is it not possible? 

MySQL delete multiple rows in one query conditions unique to each row

I&#39;m looking for a implementation of java.util.Queue or something in the Google collection who behave like a Queue, but also ensure that each element of the queue is unique. (all further insertion will have no effect)

It&#39;s that possible, or will I have to do it by hand?

For now I&#39;m using a Queue&lt;MyStuff&gt;, with a LinkedList implementation, and I check the uniqueness before insertion. ( I use a side Map for doing this, add / remove element from the side map before / after the queu ). I don&#39;t like it too much.

Any input is welcome. If it&#39;s not in the java.util package, then maybe it&#39;s a bad idea?

A Queue that ensure uniqueness of the elements?

I&#39;m trying to populate a SQL table with a list of words. The table itself it pretty simple:


    CREATE TABLE WORDS(
      ID BIGINT AUTO_INCREMENT, 
      WORD VARCHAR(128) NOT NULL UNIQUE, 
      PRIMARY KEY(ID)
    );

The problem I&#39;m running into is this: when I do the following inserts back to back

    INSERT INTO WORDS(WORD) VALUES(&#39;Seth&#39;);
    INSERT INTO WORDS(WORD) VALUES(&#39;seth&#39;);

The second insert fails with a constraint violation (&quot;Duplicate entry &#39;seth&#39; for key &#39;WORD&#39;&quot;).

How can I get the `UNIQUE` constraint on `WORD` to be case sensitive?

SQL unique varchar case sensitivity question

I&#39;ve created the following table: 

    CREATE TABLE MMCompany (
       CompanyUniqueID BIGSERIAL PRIMARY KEY NOT NULL, 
       Name VARCHAR (150) NOT NULL,
       PhoneNumber VARCHAR(20) NOT NULL UNIQUE, 
       Email VARCHAR(75) UNIQUE,
       CompanyLogo BYTEA
     );

The email column is unique and it causes a &quot;bug&quot; in my scenario since there could only be one record with null. I&#39;m trying to achieve records of companies without the same email but at the same time allow a companies to have no email.

How can I achieve that?

Allow null in unique column

I have a Table like this one:

    |UserId   |  ContactID |  ContactName 
    ---------------------------------------
    | 12456   |  Ax759     |  Joe Smith
    | 12456   |  Ax760     |  Mary Smith
    | 12458   |  Ax739     |  Carl Lewis
    | 12460   |  Ax759     |  Chuck Norris
    | 12460   |  Bx759     |  Bruce Lee

I need to add a constraint to this table so that no user can have duplicate contact id&#39;s.  The users are importing data from various external systems so ContactId will not be unique across the board but will be unique on a per user basis.

I know how to create Unique and Non-Null contraints based on single columns but how can I create a unique contraints across 2 columns?


How can I create a SQL unique constraint based on 2 columns?

Suppose the following DB migration in Ruby:

&lt;pre&gt;
    create_table :question_votes do |t|
      t.integer :user_id
      t.integer :question_id
      t.integer :vote

      t.timestamps
    end
&lt;/pre&gt;

Suppose further that I wish the rows in the DB contain unique (user_id, question_id) pairs.  What is the right dust to put in the model to accomplish that?

&lt;pre&gt;validates\_uniqueness\_of :user\_id, :question\_id&lt;/pre&gt; seems to simply make rows unique by user id, and unique by question id, instead of unique by the pair.


How do you validate uniqueness of a pair of ids in Ruby on Rails?

In dynamodb, if you want to enforce uniqueness in a field other than the primary key (like were you have a users table and want unique email addresses for users while primary key is a userid which is a number) is there a way other thans scanning the table to see if the email is already in use?  

Is there a way to enforce unique constraint on a property (field) other than the primary key in dynamodb

I am trying to insert values in table. But there is only one value inserted. I am getting an error in log cat when I am trying to insert new values. 

Log cat shows :

    abort at 13 in [INSERT INTO event(totalminutesfrom,dayofweek,title,location,totalminutesto,id) VALUES (?,?,?,?,?,?)]: UNIQUE constraint failed: event.id
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase: Error inserting totalminutesfrom=694 dayofweek=null title=qxs location=Eded &amp; Mariz totalminutesto=0 id=0
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase: android.database.sqlite.SQLiteConstraintException: UNIQUE constraint failed: event.id (code 1555)
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase:     at android.database.sqlite.SQLiteConnection.nativeExecuteForLastInsertedRowId(Native Method)
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase:     at android.database.sqlite.SQLiteConnection.executeForLastInsertedRowId(SQLiteConnection.java:782)
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase:     at android.database.sqlite.SQLiteSession.executeForLastInsertedRowId(SQLiteSession.java:788)
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase:     at android.database.sqlite.SQLiteStatement.executeInsert(SQLiteStatement.java:86)
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase:     at android.database.sqlite.SQLiteDatabase.insertWithOnConflict(SQLiteDatabase.java:1471)
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase:     at android.database.sqlite.SQLiteDatabase.insert(SQLiteDatabase.java:1341)
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase:     at com.example.siddhi.timetablelayout.EventTableHelper.addEvent(EventTableHelper.java:76)
    01-24 11:34:39.764 7763-7763/com.example.siddhi.timetablelayout E/SQLiteDatabase:     at com.example.siddhi.timetablelayout.AddEventActivity$5.onClick(AddEventActivity.java:217)


Its showing error on these two lines while inserting row.

    db.insert(TABLE, null, values);

       db.addEvent(new EventData(eventTitle,dayOfWeek,totalMinutesFrom, totalMinutesTo,location));



EventTableHelper


        
    public class EventTableHelper extends SQLiteOpenHelper {
    
    
        private static final String TABLE = &quot;event&quot;;
        private static final String KEY_ID = &quot;id&quot;;
        private static final String KEY_TITLE = &quot;title&quot;;
        private static final String KEY_LOCATION = &quot;location&quot;;
        private static final String KEY_DAY_OF_WEEK = &quot;dayofweek&quot;;
        private static final String KEY_TOTAL_MINUTES_FROM = &quot;totalminutesfrom&quot;;
        private static final String KEY_TOTAL_MINUTES_TO = &quot;totalminutesto&quot;;
    
    
    
        public EventTableHelper(Context context) {
            super(context, Constants.DATABASE_NAME, null, Constants.DATABASE_VERSION);
            //3rd argument to be passed is CursorFactory instance
        }
    
        // Creating Tables
        @Override
        public void onCreate(SQLiteDatabase db) {
            //createTable(db);
        }
    
        public void createTable(SQLiteDatabase db){
            String CREATE_EVENTS_TABLE = &quot;CREATE TABLE &quot; + TABLE + &quot;(&quot;
                    + KEY_ID + &quot; INTEGER PRIMARY KEY,&quot; + KEY_TITLE + &quot; TEXT,&quot;
                    + KEY_DAY_OF_WEEK +&quot;TEXT&quot; + KEY_TOTAL_MINUTES_FROM +&quot;INTEGER&quot;
                    + KEY_TOTAL_MINUTES_TO + &quot;INTEGER&quot; +  KEY_LOCATION + &quot;TEXT&quot; +  &quot;)&quot;;
    
            db.execSQL(CREATE_EVENTS_TABLE);
    
        }
        // Upgrading database
        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            // Drop older table if existed
           // db.execSQL(&quot;DROP TABLE IF EXISTS &quot; + TABLE);
    
          //  createTable(db);
    
            // Create tables again
            //onCreate(db);
        }
    
        // code to add the new contact
        public void addEvent(EventData event) {
            SQLiteDatabase db = this.getWritableDatabase();
    
            ContentValues values = new ContentValues();
            values.put(KEY_ID, event.getId());
            values.put(KEY_TITLE,event.getTitle()); // Contact Name
            values.put(KEY_DAY_OF_WEEK,event.getDayofWeek());
            values.put(KEY_TOTAL_MINUTES_FROM,event.getFromMinutes());
            values.put(KEY_TOTAL_MINUTES_TO,event.getToMinutes());
            values.put(KEY_LOCATION,event.getLocation());
            // Inserting Row
            db.insert(TABLE, null, values);
            //2nd argument is String containing nullColumnHack
            db.close(); // Closing database connection
        }
    
        // code to get the single contact
       EventData getEvent(int id) {
            SQLiteDatabase db = this.getReadableDatabase();
    
            Cursor cursor = db.query(TABLE, new String[] { KEY_ID,
                            KEY_TITLE, KEY_DAY_OF_WEEK, KEY_TOTAL_MINUTES_FROM,KEY_TOTAL_MINUTES_TO,KEY_LOCATION }, KEY_ID + &quot;=?&quot;,
                    new String[] { String.valueOf(id) }, null, null, null, null);
            if (cursor != null)
                cursor.moveToFirst();
           EventData eventData = new EventData(Integer.parseInt(cursor.getString(0)),cursor.getString(1), cursor.getString(2),
                   cursor.getInt(3),cursor.getInt(4),cursor.getString(5));
    
            return eventData;
        }
    
    
    
        // code to get all contacts in a list view
        public List&lt;EventData&gt; getAllEvents() {
            List&lt;EventData&gt; conList = new ArrayList&lt;EventData&gt;();
            // Select All Query
            String selectQuery = &quot;SELECT  * FROM &quot; + TABLE;
    
            SQLiteDatabase db = this.getWritableDatabase();
            Cursor cursor = db.rawQuery(selectQuery, null);
    
            // looping through all rows and adding to list
            if (cursor.moveToFirst()) {
                do {
    
                    EventData event = new EventData();
    
                    event.setId(Integer.parseInt(cursor.getString(0)));
                    event.setTitle(cursor.getString(1));
                    event.setDayofWeek(cursor.getString(2));
                    event.setFromMinutes(cursor.getInt(3));
                    event.setToMinutes(cursor.getInt(4));
                    event.setLocation(cursor.getString(5));
                    // Adding contact to list
                    conList.add(event);
                } while (cursor.moveToNext());
            }
    
            // return contact list
            return conList;
        }
    }

How to solve this??

UNIQUE constraint failed: sqlite database : android

Suppose a simple Grails domain class:

    class Account {
        String countryId;

        String userName;

        String password;

        static constraints = {
            ...???...
        }
    }

It is required that user names are unique for a particular &lt;i&gt;countryId&lt;/i&gt;, thus there must be a &lt;i&gt;unique&lt;/i&gt; contraint on two columns. How to express this in the &lt;i&gt;constraints&lt;/i&gt; definition? 

All Unique Constraint Solutions on Gang of Coders

Total of 26 Unique Constraint Solutions

How can I do 'insert if not exists' in MySQL?