fix unique key null bug

tests/null_unique_index/run.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+set -eu
+cur=$(cd `dirname $0`; pwd)
+
+DB_NAME="null_unique_key"
+
+# drop database on mysql
+run_sql "drop database if exists \`$DB_NAME\`;"
+
+# build data on mysql
+run_sql "create database \`$DB_NAME\`;"
+run_sql "create table \`$DB_NAME\`.\`t\` (a int unique key, b int);"
+run_sql "insert into \`$DB_NAME\`.\`t\` values (1, 2), (NULL, 1);"
+
+
+# dumping
+export DUMPLING_TEST_DATABASE=$DB_NAME
+run_dumpling -r 1
+
+data="NULL"
+cnt=$(sed "s/$data/$data\n/g" $DUMPLING_OUTPUT_DIR/$DB_NAME.t.1.sql | grep -c "$data") || true
+[ $cnt = 1 ]
+

v4/export/ir_impl.go

@@ -323,16 +323,20 @@ func splitTableDataIntoChunks(
 	}
 
 	chunkIndex := 0
+	nullValueCondition := fmt.Sprintf("`%s` IS NULL OR ", escapeString(field))
 LOOP:
 	for cutoff <= max {
 		chunkIndex += 1
-		where := fmt.Sprintf("(`%s` >= %d AND `%s` < %d)", escapeString(field), cutoff, escapeString(field), cutoff+estimatedStep)
+		where := fmt.Sprintf("%s(`%s` >= %d AND `%s` < %d)", nullValueCondition, escapeString(field), cutoff, escapeString(field), cutoff+estimatedStep)
 		query = buildSelectQuery(dbName, tableName, selectedField, buildWhereCondition(conf, where), orderByClause)
 		rows, err := db.Query(query)
 		if err != nil {
 			errCh <- errors.WithMessage(err, query)
 			return
 		}
+		if len(nullValueCondition) > 0 {
+			nullValueCondition = ""
+		}
 
 		td := &tableData{
 			database:      dbName,