add support for IF NOT EXISTS in ALTER TABLE

h2/src/docsrc/help/help.csv

@@ -208,7 +208,7 @@ ALTER SEQUENCE SEQ_ID RESTART WITH 1000
 "
 
 "Commands (DDL)","ALTER TABLE ADD","
-ALTER TABLE tableName ADD name dataType [ DEFAULT expression ]
+ALTER TABLE tableName ADD [ IF NOT EXISTS ] name dataType [ DEFAULT expression ]
 [ [ NOT ] NULL ] [ AUTO_INCREMENT | IDENTITY ] [ BEFORE columnName ]
 ","
 Adds a new column to a table.

h2/src/docsrc/html/changelog.html

@@ -34,6 +34,7 @@ Change Log
 </li><li>Linked tables: the index conditions was sometimes not used when querying the remote database.
 </li><li>Issue 294: OSGi: the versions were missing in manifest package exports.
 </li><li>The option -baseDir didn't work with symbolic links.
+</li><li>Support for ALTER TABLE ADD COLUMN IF NOT EXISTS.
 </li><li>Database-level connection settings could only be set in the database URL,
     but not using the Properties parameter of DriverManager.getConnection(String url, Properties info).
 </li></ul>

h2/src/docsrc/html/roadmap.html

@@ -505,7 +505,6 @@ See also <a href="build.html#providing_patches">Providing Patches</a>.
     disable autocommit for all connections.
 </li><li>Compatibility with MS Access: support "&amp;" to concatenate text.
 </li><li>The BACKUP statement should not synchronize on the database, and therefore should not block other users.
-</li><li>Support ALTER TABLE ADD COLUMN IF NOT EXISTS.
 </li><li>Document the database file format.
 </li><li>Support reading LOBs.
 </li><li>Require appending DANGEROUS=TRUE when using certain dangerous settings such as

h2/src/main/org/h2/command/CommandInterface.java

@@ -447,6 +447,11 @@ public interface CommandInterface {
      */
     int SHUTDOWN_DEFRAG = 84;
 
+    /**
+     * The type of a ALTER TABLE ADD IF NOT EXISTS statement.
+     */
+    int ALTER_TABLE_ADD_COLUMN_IF_NOT_EXISTS = 85;
+    
     /**
      * Get command type.
      *

h2/src/main/org/h2/command/Parser.java

@@ -4778,9 +4778,10 @@ public class Parser {
 
     private AlterTableAlterColumn parseAlterTableAddColumn(Table table) {
         readIf("COLUMN");
+        boolean ifNotExists = readIfNoExists();
         Schema schema = table.getSchema();
         AlterTableAlterColumn command = new AlterTableAlterColumn(session, schema);
-        command.setType(CommandInterface.ALTER_TABLE_ADD_COLUMN);
+        command.setType(ifNotExists ? CommandInterface.ALTER_TABLE_ADD_COLUMN_IF_NOT_EXISTS : CommandInterface.ALTER_TABLE_ADD_COLUMN);
         command.setTable(table);
         String columnName = readColumnIdentifier();
         Column column = parseColumnForTable(columnName, true);

h2/src/main/org/h2/command/ddl/AlterTableAlterColumn.java

@@ -35,6 +35,7 @@ import org.h2.util.New;
 /**
  * This class represents the statements
  * ALTER TABLE ADD,
+ * ALTER TABLE ADD IF NOT EXISTS,
  * ALTER TABLE ALTER COLUMN,
  * ALTER TABLE ALTER COLUMN RESTART,
  * ALTER TABLE ALTER COLUMN SELECTIVITY,
@@ -122,6 +123,13 @@ public class AlterTableAlterColumn extends SchemaCommand {
             copyData();
             break;
         }
+        case CommandInterface.ALTER_TABLE_ADD_COLUMN_IF_NOT_EXISTS: {
+            if (!table.doesColumnExist(newColumn.getName())) {
+                convertAutoIncrementColumn(newColumn);
+                copyData();
+            }
+            break;
+        }
         case CommandInterface.ALTER_TABLE_DROP_COLUMN: {
             if (table.getColumns().length == 1) {
                 throw DbException.get(ErrorCode.CANNOT_DROP_LAST_COLUMN, oldColumn.getSQL());
@@ -216,7 +224,8 @@ public class AlterTableAlterColumn extends SchemaCommand {
         if (type == CommandInterface.ALTER_TABLE_DROP_COLUMN) {
             int position = oldColumn.getColumnId();
             newColumns.remove(position);
-        } else if (type == CommandInterface.ALTER_TABLE_ADD_COLUMN) {
+        } else if (type == CommandInterface.ALTER_TABLE_ADD_COLUMN
+                || type == CommandInterface.ALTER_TABLE_ADD_COLUMN_IF_NOT_EXISTS) {
             int position;
             if (addBefore == null) {
                 position = columns.length;
@@ -254,7 +263,8 @@ public class AlterTableAlterColumn extends SchemaCommand {
             if (columnList.length() > 0) {
                 columnList.append(", ");
             }
-            if (type == CommandInterface.ALTER_TABLE_ADD_COLUMN && nc == newColumn) {
+            if ((type == CommandInterface.ALTER_TABLE_ADD_COLUMN 
+                || type == CommandInterface.ALTER_TABLE_ADD_COLUMN_IF_NOT_EXISTS) && nc == newColumn) {
                 Expression def = nc.getDefaultExpression();
                 columnList.append(def == null ? "NULL" : def.getSQL());
             } else {

h2/src/main/org/h2/res/help.csv

@@ -76,7 +76,7 @@ ALTER SEQUENCE sequenceName [ RESTART WITH long ] [ INCREMENT BY long ]
 ","
 Changes the next value and the increment of a sequence."
 "Commands (DDL)","ALTER TABLE ADD","
-ALTER TABLE tableName ADD name dataType [ DEFAULT expression ]
+ALTER TABLE tableName ADD [ IF NOT EXISTS ] name dataType [ DEFAULT expression ]
 [ [ NOT ] NULL ] [ AUTO_INCREMENT | IDENTITY ] [ BEFORE columnName ]
 ","
 Adds a new column to a table."

h2/src/main/org/h2/table/Table.java

@@ -589,6 +589,16 @@ public abstract class Table extends SchemaObjectBase {
         return column;
     }
 
+    /**
+     * Does the column with the given name exist?
+     *
+     * @param columnName the column name
+     * @return true if the column exists
+     */
+    public boolean doesColumnExist(String columnName) {
+        return columnMap.containsKey(columnName);
+    }
+
     /**
      * Get the best plan for the given search mask.
      *

h2/src/test/org/h2/test/db/TestAlter.java

@@ -38,6 +38,7 @@ public class TestAlter extends TestBase {
         testAlterTableDropColumnWithReferences();
         testAlterTableAlterColumn();
         testAlterTableDropIdentityColumn();
+        testAlterTableAddColumnIfNotExists();
         conn.close();
         deleteDb("alter");
     }
@@ -123,4 +124,14 @@ public class TestAlter extends TestBase {
         stat.execute("drop table t");
     }
 
+    private void testAlterTableAddColumnIfNotExists() throws SQLException {
+        stat.execute("create table t(x varchar) as select 'x'");
+        stat.execute("alter table t add if not exists x int");
+        stat.execute("drop table t");
+        stat.execute("create table t(x varchar) as select 'x'");
+        stat.execute("alter table t add if not exists y int");
+        stat.execute("select x, y from t");
+        stat.execute("drop table t");
+    }
+
 }