commit 0a232dab548e6e29545a00ddc2876ef4f4568874
Author: yuxianjiang <yuxianjiang@tencent.com>
Date:   Mon Aug 9 12:19:28 2021 +0800

    [bugfix] show create table error on json field
    
    Problem
    ====
    After setting sql_mode to 0, SQL statement 'create table tjson7(f1 int, f2 json
    not null default '');' can be executed successfully. But retrieving create table
    SQL by show create table, the SQL will fail whatever sql_mode is stetted. And
    mysqldump will fail when import data. By checking the statement, we found that
    definition of " `f2` json NOT NULL DEFAULT 'null'" will cause stop creating table successfully.
    
    Solution
    ====
    When json field is not null and has default value of 'null' by retrieving json
    default value. Convert the result  'null' to ''.

diff --git a/mysql-test/r/bugfix_show_create_table_json.result b/mysql-test/r/bugfix_show_create_table_json.result
new file mode 100644
index 0000000..f799723
--- /dev/null
+++ b/mysql-test/r/bugfix_show_create_table_json.result
@@ -0,0 +1,15 @@
+#Test for show
+SET sql_mode = 0;
+Warnings:
+Warning	3090	Changing sql mode 'NO_AUTO_CREATE_USER' is deprecated. It will be removed in a future release.
+create database tdb1;
+use tdb1;
+create table tjson1(f1 int, f2 json not null default '');
+Warnings:
+Warning	1101	BLOB, TEXT, GEOMETRY or JSON column 'f2' can't have a default value
+
+#clean up and reset enviroment
+
+drop table tjson1;
+drop database tdb1;
+SET sql_mode = default;
diff --git a/mysql-test/t/bugfix_show_create_table_json.test b/mysql-test/t/bugfix_show_create_table_json.test
new file mode 100644
index 0000000..c045dac
--- /dev/null
+++ b/mysql-test/t/bugfix_show_create_table_json.test
@@ -0,0 +1,20 @@
+#
+# Test of show create table on json field. When json field is created
+# with not null default '', show create table should also return the
+# same format.
+#
+
+--echo #Test for show
+SET sql_mode = 0;
+
+create database tdb1;
+use tdb1;
+create table tjson1(f1 int, f2 json not null default '');
+
+
+--echo
+--echo #clean up and reset enviroment
+--echo
+drop table tjson1;
+drop database tdb1;
+SET sql_mode = default;
diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index 2ee3deb..c1cc7db 100644
--- a/sql/sql_show.cc
+++ b/sql/sql_show.cc
@@ -1432,20 +1432,30 @@ static bool print_default_clause(THD *thd, Field *field, String *def_value,
       }
       else
         field->val_str(&type);
-      if (type.length())
+      if (field_type == MYSQL_TYPE_JSON && type.length()
+          && strcmp("null", type.ptr()) == 0)
       {
-        String def_val;
-        uint dummy_errors;
-        /* convert to system_charset_info == utf8 */
-        def_val.copy(type.ptr(), type.length(), field->charset(),
-                     system_charset_info, &dummy_errors);
-        if (quoted)
-          append_unescaped(def_value, def_val.ptr(), def_val.length());
-        else
-          def_value->append(def_val.ptr(), def_val.length());
+        /* for column defination of json not null default '' under sql_mode = 0 */
+        DBUG_ASSERT(has_default && !field->is_null() && field_type == MYSQL_TYPE_JSON);
+        append_unescaped(def_value, "", 0);
+      }
+      else
+      {
+        if (type.length())
+        {
+          String def_val;
+          uint dummy_errors;
+          /* convert to system_charset_info == utf8 */
+          def_val.copy(type.ptr(), type.length(), field->charset(),
+                       system_charset_info, &dummy_errors);
+          if (quoted)
+            append_unescaped(def_value, def_val.ptr(), def_val.length());
+          else
+            def_value->append(def_val.ptr(), def_val.length());
+        }
+        else if (quoted)
+          def_value->append(STRING_WITH_LEN("''"));
       }
-      else if (quoted)
-        def_value->append(STRING_WITH_LEN("''"));
     }
     else if (field->maybe_null() && quoted)
       def_value->append(STRING_WITH_LEN("NULL"));    // Null as default