Support Alter Table Drop Constraint If Exists (#709)

* Support Alter Table Drop Constraint If Exists

* #709 add constraintIfExists flag

Author: rfscholte

src/main/java/net/sf/jsqlparser/statement/alter/AlterExpression.java

@@ -47,6 +47,7 @@ public class AlterExpression {
     private String ukName;
     private Index index = null;
     private String constraintName;
+    private boolean constraintIfExists;
     private boolean onDeleteRestrict;
     private boolean onDeleteSetNull;
     private boolean onDeleteCascade;
@@ -143,6 +144,14 @@ public String getConstraintName() {
     public void setConstraintName(final String constraintName) {
         this.constraintName = constraintName;
     }
+    
+    public boolean isConstraintIfExists() {
+        return constraintIfExists;
+    }
+    
+    public void setConstraintIfExists( boolean constraintIfExists ) {
+        this.constraintIfExists = constraintIfExists;
+    }
 
     public List<String> getPkColumns() {
         return pkColumns;
@@ -215,7 +224,11 @@ public String toString() {
                 b.append(")");
             }
         } else if (constraintName != null) {
-            b.append("CONSTRAINT ").append(constraintName);
+            b.append("CONSTRAINT ");
+            if(constraintIfExists) {
+               b.append("IF EXISTS ");
+            }
+            b.append(constraintName);
         } else if (pkColumns != null) {
             b.append("PRIMARY KEY (").append(PlainSelect.getStringList(pkColumns)).append(')');
         } else if (ukColumns != null) {

src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt

@@ -3864,7 +3864,7 @@ AlterExpression AlterExpression():
 
              )
              |
-       (  <K_CONSTRAINT>
+       (  <K_CONSTRAINT> [<K_IF> <K_EXISTS> { alterExp.setConstraintIfExists(true); } ]
        (tk=<S_IDENTIFIER>    | tk=<S_QUOTED_IDENTIFIER>)
            {
               alterExp.setConstraintName(tk.image);

src/test/java/net/sf/jsqlparser/statement/alter/AlterTest.java

@@ -146,6 +146,11 @@ public void testAlterTableDropConstraint() throws JSQLParserException {
         assertEquals(alterExpression.getConstraintName(), "YYY");
     }
 
+    @Test
+    public void testAlterTableDropConstraintIfExists() throws JSQLParserException {
+        assertSqlCanBeParsedAndDeparsed("ALTER TABLE Persons DROP CONSTRAINT IF EXISTS UC_Person");
+    }
+
     @Test
     public void testAlterTablePK() throws JSQLParserException {
         final String sql = "ALTER TABLE `Author` ADD CONSTRAINT `AuthorPK` PRIMARY KEY (`ID`)";