@Select 与 @SelectProvider 只是在定义注解的方式上有所不同, 一个是静态SQL, 一个是动态SQL 。

@SelectProvider 是 MyBatis 中的一个注解，用于指定一个类或者类的某个方法提供 SQL 查询语句。该注解常用于动态 SQL 的场景，例如根据不同的参数生成不同的查询语句。 使用 @SelectProvider 注解的方式可以让 MyBatis 在运行时根据注解指定的类或方法来生成对应的 SQL 查询语句，从而实现动态 SQL 功能。注解的语法如下：

@SelectProvider(type = XxxProvider.class, method = "xxxMethod")

其中，type 属性表示提供 SQL 查询语句的类，method 属性表示类中提供查询语句的方法。在查询时，MyBatis 会调用指定类中的指定方法来生成 SQL 查询语句，然后执行该查询语句并返回结果。 需要注意的是，提供 SQL 查询语句的类需要实现 org.apache.ibatis.builder.annotation.ProviderMethodResolver 接口，该接口中定义了查找提供 SQL 查询的方法的逻辑。同时，提供查询语句的方法需要返回一个字符串类型的 SQL 查询语句。 使用 @SelectProvider 注解可以让 MyBatis 的查询语句更加灵活，适用于各种复杂的查询场景。

1 @SelectProvider 使用

1 TestMapper.java

public interface TestMapper {

    /**
     * 根据性别获取老师信息
     *
     * @param sex
     * @return
     * @date 2022年11月26日12点09分
     */
    @SelectProvider(type = Test.class, method = "list")
    List<Teacher> list(Integer sex);

}

2 Test.java

public class Test {

    /**
     * 根据性别获取老师信息
     *
     * @param sex
     * @return
     * @date 2022年11月26日12点09分
     */
    public String list(Integer sex) {
        return new SQL() {
            {
                SELECT("*");
                FROM("teacher");
                if (null != sex) {

                    WHERE(" 1 = 1 sex = " + sex);

                } else {
                    WHERE(" 1 = 1 ");
                }
                ORDER_BY("create_time desc");
            }
        }.toString();
    }

}

2 模拟 Mybatis Plus

可以使用 SelectProvider 写一个适用于所有表的查询的方法。

1 BasicWrapper.java

import java.util.LinkedList;
import java.util.List;

/**
 * @author Administrator
 */
public class BasicWrapper<T> {

    public String last;

    public Class<T> bean;

    public String table;

    public String[] field;

    public List<String> condition = new LinkedList<>();

    public String orderBy;

}

2 QueryWrapper.java

import java.util.Arrays;

import org.apache.ibatis.jdbc.SQL;

import cn.hutool.core.collection.CollectionUtil;

/**
 * @author 
 */
public class QueryWrapper<T> extends BasicWrapper {

    public QueryWrapper() {
    }

    public QueryWrapper(String table, String... field) {
        super.bean = null;
        super.table = table;
        super.field = (null == field || field.length == 0) ? new String[]{"*"} : field;
    }

    public String list(QueryWrapper wrapper) {

        Query query = wrapper.build();

        return new SQL() {
            {
                SELECT(query.getFields());
                FROM(query.getTable());
                WHERE(" 1 = 1 " + query.getCondition());
                ORDER_BY(query.getOrderBy());
            }
        }.toString();

    }

    public Query build() {
        Query query = new Query();
        query.setTable(super.table);
        query.setFields((null == super.field || super.field.length == 0) ? "*" : String.join(", ", Arrays.asList(super.field)));
        String condition = CollectionUtil.isEmpty(super.condition) ? " 1 = 1 " : String.join(" ", super.condition);
        String last = null == super.last ? "" : super.last;
        query.setCondition(condition + " " + last);
        return query;
    }

    public QueryWrapper orderBy(String sql) {
        super.orderBy = sql;
        return this;
    }

    public QueryWrapper orderBy(boolean condition, String sql) {
        if (condition) {
            super.orderBy = sql;
        }
        return this;
    }

    public QueryWrapper apply(String sql) {
        super.condition.add(" and (" + sql + ") ");
        return this;
    }

    public QueryWrapper apply(boolean condition, String sql) {
        if (condition) {
            super.condition.add(" and (" + sql + ") ");
        }
        return this;
    }

    public QueryWrapper eq(String filed, Object value) {
        super.condition.add(" and " + filed + " = " + value);
        return this;
    }

    public QueryWrapper eq(boolean condition, String filed, Object value) {
        if (condition) {
            if (value instanceof String) {
                super.condition.add(" and " + filed + " = '" + value + "'");
            } else {
                super.condition.add(" and " + filed + " = " + value);
            }
        }
        return this;
    }

    public QueryWrapper last(String value) {
        super.last = " " + value;
        return this;
    }

    public QueryWrapper last(boolean condition, String value) {
        if (condition) {
            super.last = " " + value;
        }
        return this;
    }

    public QueryWrapper like(String filed, Object value) {
        super.condition.add(" and " + filed + " like %" + value + "%");
        return this;
    }

    public QueryWrapper likeLeft(String filed, Object value) {
        super.condition.add(" and " + filed + " like %" + value + "%");
        return this;
    }

    public QueryWrapper likeRight(String filed, Object value) {
        super.condition.add(" and " + filed + " like %" + value + "%");
        return this;
    }

    public QueryWrapper like(boolean condition, String filed, Object value) {
        if (condition) {
            super.condition.add(" and " + filed + " like %" + value + "%");
        }
        return this;
    }

    public QueryWrapper likeLeft(boolean condition, String filed, Object value) {
        if (condition) {
            super.condition.add(" and " + filed + " like %" + value + "%");
        }
        return this;
    }

    public QueryWrapper likeRight(boolean condition, String filed, Object value) {
        if (condition) {
            super.condition.add(" and " + filed + " like %" + value + "%");
        }
        return this;
    }

}

3 Query.java

import lombok.Data;

/**
 * @author Administrator
 */
@Data
public class Query {

    private String table;

    private String fields;

    private String orderBy;

    private String condition;

}

4 Test.java

import org.apache.ibatis.jdbc.SQL;

public class Test {
	public static void main(String agrs[]) {
		    QueryWrapper<Teacher> wrapper = new QueryWrapper<>("teacher");
            wrapper.eq("sex", 1);
            wrapper.eq(StringUtils.isNotBlank(name), "name", name);
            wrapper.orderBy("create_time desc");
            List<Teacher> list = processMapper.list(wrapper);
	}

}