add convert method; close #I781LP

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

@@ -65,6 +65,11 @@ public class QueryMethods {
         return new DistinctQueryColumn(columns);
     }
 
+    //CONVERT ( data_type [ ( length ) ] , expression [ , style ] )
+    public static StringFunctionQueryColumn convert(String... params) {
+        return new StringFunctionQueryColumn("CONVERT", params);
+    }
+
     public static StringQueryColumn column(String column) {
         return new StringQueryColumn(column);
     }

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

@@ -0,0 +1,83 @@
+/**
+ * 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.Arrays;
+import java.util.List;
+
+/**
+ * 数据库 聚合函数，例如 CONVERT(NVARCHAR(30), GETDATE(), 126) 等等
+ */
+public class StringFunctionQueryColumn extends QueryColumn {
+
+    protected String fnName;
+    protected List<String> params;
+
+    public StringFunctionQueryColumn(String fnName, String ...params) {
+        SqlUtil.keepColumnSafely(fnName);
+        this.fnName = fnName;
+        this.params = Arrays.asList(params);
+    }
+
+
+    public String getFnName() {
+        return fnName;
+    }
+
+    public void setFnName(String fnName) {
+        this.fnName = fnName;
+    }
+
+
+    public List<String> getParams() {
+        return params;
+    }
+
+    public void setParams(List<String> params) {
+        this.params = params;
+    }
+
+    @Override
+    public String toSelectSql(List<QueryTable> queryTables, IDialect dialect) {
+        String sql = StringUtil.join(", ",params);
+        return StringUtil.isBlank(sql) ? "" : fnName + "(" + sql + ")" + WrapperUtil.buildAsAlias(alias);
+    }
+
+    @Override
+    String toConditionSql(List<QueryTable> queryTables, IDialect dialect) {
+        String sql = StringUtil.join(", ",params);
+        return StringUtil.isBlank(sql) ? "" : fnName + "(" + sql + ")";
+    }
+
+    @Override
+    public QueryColumn as(String alias) {
+        SqlUtil.keepColumnSafely(alias);
+        this.alias = alias;
+        return this;
+    }
+
+    @Override
+    public String toString() {
+        return "StringFunctionQueryColumn{" +
+                "fnName='" + fnName + '\'' +
+                ", params=" + params +
+                '}';
+    }
+}

mybatis-flex-core/src/test/java/com/mybatisflex/coretest/AccountSqlTester.java

@@ -306,6 +306,22 @@ public class AccountSqlTester {
         System.out.println(sql);
     }
 
+    @Test
+    public void testConvert() {
+        IDialect dialect = new CommonsDialectImpl();
+
+        QueryWrapper queryWrapper = QueryWrapper.create()
+                .select(ACCOUNT.ALL_COLUMNS, convert("NVARCHAR(30)", "GETDATE()", "126").as("result"))
+                .from(ACCOUNT)
+                .and(ACCOUNT.USER_NAME.like("michael"))
+                .and(convert("NVARCHAR(30)", "GETDATE()", "126").in(
+                        select(ACCOUNT.ID).from(ACCOUNT).where(ACCOUNT.ID.ge(100)))
+                );
+
+        String sql = dialect.forSelectByQuery(queryWrapper);
+        System.out.println(sql);
+    }
+
 
     @Test
     public void testLimitOffset() {