feat: add SelectQueryColumn and close https://gitee.com/mybatis-flex/mybatis-flex/issues/I76RVU

mybatis-flex-core/src/main/java/com/mybatisflex/core/query/QueryMethods.java

@@ -65,11 +65,14 @@ public class QueryMethods {
         return new DistinctQueryColumn(columns);
     }
 
-
     public static StringQueryColumn column(String column) {
         return new StringQueryColumn(column);
     }
 
+    public static SelectQueryColumn column(QueryWrapper queryWrapper) {
+        return new SelectQueryColumn(queryWrapper);
+    }
+
     public static QueryCondition exists(QueryWrapper queryWrapper) {
         return new OperatorSelectCondition(" EXISTS ", queryWrapper);
     }
@@ -95,11 +98,18 @@ public class QueryMethods {
         return newWrapper().select(queryColumns);
     }
 
-
     public static QueryWrapper selectOne() {
         return select(column("1"));
     }
 
+    public static QueryWrapper selectCount() {
+        return select(count());
+    }
+
+    public static QueryWrapper selectCountOne(String countP) {
+        return select(count("1"));
+    }
+
     public static RawValue raw(String raw){
         return new RawValue(raw);
     }

mybatis-flex-core/src/main/java/com/mybatisflex/core/query/SelectQueryColumn.java

@@ -0,0 +1,53 @@
+/**
+ * Copyright (c) 2022-2023, Mybatis-Flex (fuhai999@gmail.com).
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.mybatisflex.core.query;
+
+import com.mybatisflex.core.dialect.IDialect;
+import com.mybatisflex.core.util.SqlUtil;
+
+import com.mybatisflex.core.util.StringUtil;
+
+import java.util.List;
+
+public class SelectQueryColumn extends QueryColumn {
+
+    private QueryWrapper queryWrapper;
+
+    public SelectQueryColumn(QueryWrapper queryWrapper) {
+        this.queryWrapper = queryWrapper;
+    }
+
+    @Override
+    public SelectQueryColumn as(String alias) {
+        SqlUtil.keepColumnSafely(alias);
+        this.alias = alias;
+        return this;
+    }
+
+    @Override
+    String toSelectSql(List<QueryTable> queryTables, IDialect dialect) {
+        String selectSql = dialect.forSelectByQuery(queryWrapper);
+        if (StringUtil.isNotBlank(selectSql) && StringUtil.isNotBlank(alias)) {
+            selectSql = "(" + selectSql + ") AS " + dialect.wrap(alias);
+        }
+        return selectSql;
+    }
+
+    @Override
+    String toConditionSql(List<QueryTable> queryTables, IDialect dialect) {
+        return super.toConditionSql(queryTables, dialect);
+    }
+}