MDEV-14694 ALTER COLUMN IF EXISTS .. causes syntax error.

Implementing the 'IF EXISTS' option for statements
        ALTER TABLE ALTER COLUMN SET/DROP DEFAULT.

mysql-test/r/alter_table.result

@@ -2275,6 +2275,19 @@ t1	CREATE TABLE `t1` (
   `c` int(11) DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
+create table t1 (i int);
+alter table t1 alter column if exists a set default 1;
+Warnings:
+Note	1054	Unknown column 'a' in 't1'
+alter table t1 alter column if exists a drop default;
+Warnings:
+Note	1054	Unknown column 'a' in 't1'
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `i` int(11) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop table t1;
 #
 # End of 10.2 tests
 #

mysql-test/t/alter_table.test

@@ -1882,6 +1882,16 @@ alter table t1 drop column a, drop index a;
 show create table t1;
 drop table t1;
 
+#
+# MDEV-14694 ALTER COLUMN IF EXISTS .. causes syntax error
+#
+
+create table t1 (i int);
+alter table t1 alter column if exists a set default 1;
+alter table t1 alter column if exists a drop default;
+show create table t1;
+drop table t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/sql_class.h

@@ -284,8 +284,9 @@ class Alter_column :public Sql_alloc {
 public:
   const char *name;
   Virtual_column_info *default_value;
-  Alter_column(const char *par_name, Virtual_column_info *expr)
+  bool alter_if_exists;
-    :name(par_name), default_value(expr) {}
+  Alter_column(const char *par_name, Virtual_column_info *expr, bool par_exists)
+    :name(par_name), default_value(expr), alter_if_exists(par_exists) {}
   /**
     Used to make a clone of this object for ALTER/CREATE TABLE
     @sa comment for Key_part_spec::clone

sql/sql_lex.h

@@ -3645,6 +3645,8 @@ public:
     alter_info.check_constraint_list.push_back(constr);
     return false;
   }
+  bool add_alter_list(const char *par_name, Virtual_column_info *expr,
+                      bool par_exists);
   void set_command(enum_sql_command command,
                    DDL_options_st options)
   {

sql/sql_table.cc

@@ -6114,6 +6114,40 @@ drop_create_field:
     }
   }
 
+  /* Handle ALTER COLUMN IF EXISTS SET/DROP DEFAULT. */
+  {
+    List_iterator<Alter_column> it(alter_info->alter_list);
+    Alter_column *acol;
+
+    while ((acol=it++))
+    {
+      if (!acol->alter_if_exists)
+        continue;
+      /*
+         If there is NO field with the same name in the table already,
+         remove the acol from the list.
+      */
+      for (f_ptr=table->field; *f_ptr; f_ptr++)
+      {
+        if (my_strcasecmp(system_charset_info,
+                           acol->name, (*f_ptr)->field_name.str) == 0)
+          break;
+      }
+      if (*f_ptr == NULL)
+      {
+        push_warning_printf(thd, Sql_condition::WARN_LEVEL_NOTE,
+                            ER_BAD_FIELD_ERROR,
+                            ER_THD(thd, ER_BAD_FIELD_ERROR),
+                            acol->name, table->s->table_name.str);
+        it.remove();
+        if (alter_info->alter_list.is_empty())
+        {
+          alter_info->flags&= ~(Alter_info::ALTER_CHANGE_COLUMN_DEFAULT);
+        }
+      }
+    }
+  }
+
   /* Handle DROP COLUMN/KEY IF EXISTS. */
   {
     List_iterator<Alter_drop> drop_it(alter_info->drop_list);
@@ -6142,7 +6176,9 @@ drop_create_field:
       }
       else if (drop->type == Alter_drop::CHECK_CONSTRAINT)
       {
-        for (uint i=table->s->field_check_constraints; i < table->s->table_check_constraints; i++)
+        for (uint i=table->s->field_check_constraints;
+             i < table->s->table_check_constraints;
+             i++)
         {
           if (my_strcasecmp(system_charset_info, drop->name,
                             table->check_constraints[i]->name.str) == 0)

sql/sql_yacc.yy

@@ -699,6 +699,18 @@ void LEX::add_key_to_list(LEX_CSTRING *field_name,
   alter_info.key_list.push_back(key, mem_root);
 }
 
+bool LEX::add_alter_list(const char *name, Virtual_column_info *expr,
+                         bool exists)
+{
+  MEM_ROOT *mem_root= thd->mem_root;
+  Alter_column *ac= new (mem_root) Alter_column(name, expr, exists);
+  if (ac == NULL)
+    return true;
+  alter_info.alter_list.push_back(ac, mem_root);
+  alter_info.flags|= Alter_info::ALTER_CHANGE_COLUMN_DEFAULT;
+  return false;
+}
+
 void LEX::init_last_field(Column_definition *field,
                           const LEX_CSTRING *field_name,
                           const CHARSET_INFO *cs)
@@ -8174,24 +8186,15 @@ alter_list_item:
             lex->alter_info.keys_onoff= Alter_info::ENABLE;
             lex->alter_info.flags|= Alter_info::ALTER_KEYS_ONOFF;
           }
-        | ALTER opt_column field_ident SET DEFAULT column_default_expr
+        | ALTER opt_column opt_if_exists_table_element field_ident SET DEFAULT column_default_expr
           {
-            LEX *lex=Lex;
+            if (Lex->add_alter_list($4.str, $7, $3))
-            Alter_column *ac= new (thd->mem_root) Alter_column($3.str,$6);
-            if (ac == NULL)
               MYSQL_YYABORT;
-            lex->alter_info.alter_list.push_back(ac, thd->mem_root);
-            lex->alter_info.flags|= Alter_info::ALTER_CHANGE_COLUMN_DEFAULT;
           }
-        | ALTER opt_column field_ident DROP DEFAULT
+        | ALTER opt_column opt_if_exists_table_element field_ident DROP DEFAULT
           {
-            LEX *lex=Lex;
+            if (Lex->add_alter_list($4.str, (Virtual_column_info*) 0, $3))
-            Alter_column *ac= (new (thd->mem_root)
-                               Alter_column($3.str, (Virtual_column_info*) 0));
-            if (ac == NULL)
               MYSQL_YYABORT;
-            lex->alter_info.alter_list.push_back(ac, thd->mem_root);
-            lex->alter_info.flags|= Alter_info::ALTER_CHANGE_COLUMN_DEFAULT;
           }
         | RENAME opt_to table_ident
           {

sql/sql_yacc_ora.yy

@@ -7735,24 +7735,15 @@ alter_list_item:
             lex->alter_info.keys_onoff= Alter_info::ENABLE;
             lex->alter_info.flags|= Alter_info::ALTER_KEYS_ONOFF;
           }
-        | ALTER opt_column field_ident SET DEFAULT column_default_expr
+        | ALTER opt_column opt_if_exists_table_element field_ident SET DEFAULT column_default_expr
           {
-            LEX *lex=Lex;
+            if (Lex->add_alter_list($4.str, $7, $3))
-            Alter_column *ac= new (thd->mem_root) Alter_column($3.str,$6);
-            if (ac == NULL)
               MYSQL_YYABORT;
-            lex->alter_info.alter_list.push_back(ac, thd->mem_root);
-            lex->alter_info.flags|= Alter_info::ALTER_CHANGE_COLUMN_DEFAULT;
           }
-        | ALTER opt_column field_ident DROP DEFAULT
+        | ALTER opt_column opt_if_exists_table_element field_ident DROP DEFAULT
           {
-            LEX *lex=Lex;
+            if (Lex->add_alter_list($4.str, (Virtual_column_info*) 0, $3))
-            Alter_column *ac= (new (thd->mem_root)
-                               Alter_column($3.str, (Virtual_column_info*) 0));
-            if (ac == NULL)
               MYSQL_YYABORT;
-            lex->alter_info.alter_list.push_back(ac, thd->mem_root);
-            lex->alter_info.flags|= Alter_info::ALTER_CHANGE_COLUMN_DEFAULT;
           }
         | RENAME opt_to table_ident
           {