mybatis_user_guide(7) SQL语句构建器类

【0】README

1）本文全文总结于 http://www.mybatis.org/mybatis-3/zh/statement-builders.html

【1】在Java代码中来动态生成SQL代码就是一场噩梦

1）problem+solutions：

1.1）problem：Java程序员面对的最痛苦的事情之一就是在Java代码中嵌入SQL语句。这么来做通常是由于SQL语句需要动态来生成-否则可以将它们放到外部文件或者存储过程中。正如你已经看到的那样，MyBatis在它的XML映射特性中有一个强大的动态SQL生成方案。但有时在Java代码内部创建SQL语句也是必要的。（干货——事实上，在Java代码中来动态生成SQL代码就是一场噩梦）
<pre name="code" class="java">String sql = "SELECT P.ID, P.USERNAME, P.PASSWORD, P.FULL_NAME, "
"P.LAST_NAME,P.CREATED_ON, P.UPDATED_ON " +
"FROM PERSON P, ACCOUNT A " +
"INNER JOIN DEPARTMENT D on D.ID = P.DEPARTMENT_ID " +
"INNER JOIN COMPANY C on D.COMPANY_ID = C.ID " +
"WHERE (P.ID = A.ID AND P.FIRST_NAME like ?) " +
"OR (P.LAST_NAME like ?) " +
"GROUP BY P.ID " +
"HAVING (P.LAST_NAME like ?) " +
"OR (P.FIRST_NAME like ?) " +
"ORDER BY P.ID, P.FULL_NAME";

1.2）solutions）MyBatis 3提供了方便的工具类来帮助解决该问题。使用SQL类，简单地创建一个实例来调用方法生成SQL语句。上面示例中的问题就像重写SQL类那样：

private String selectPersonSql() {return new SQL() {{SELECT("P.ID, P.USERNAME, P.PASSWORD, P.FULL_NAME");SELECT("P.LAST_NAME, P.CREATED_ON, P.UPDATED_ON");FROM("PERSON P");FROM("ACCOUNT A");INNER_JOIN("DEPARTMENT D on D.ID = P.DEPARTMENT_ID");INNER_JOIN("COMPANY C on D.COMPANY_ID = C.ID");WHERE("P.ID = A.ID");WHERE("P.FIRST_NAME like ?");OR();WHERE("P.LAST_NAME like ?");GROUP_BY("P.ID");HAVING("P.LAST_NAME like ?");OR();HAVING("P.FIRST_NAME like ?");ORDER_BY("P.ID");ORDER_BY("P.FULL_NAME");}}.toString();
}

【2】SQL类

1）这里给出一些示例：

// Anonymous inner class
public String deletePersonSql() {return new SQL() {{DELETE_FROM("PERSON");WHERE("ID = #{id}");}}.toString();
}// Builder / Fluent style
public String insertPersonSql() {String sql = new SQL().INSERT_INTO("PERSON").VALUES("ID, FIRST_NAME", "#{id}, #{firstName}").VALUES("LAST_NAME", "#{lastName}").toString();return sql;
}// With conditionals (note the final parameters, required for the anonymous inner class to access them)
public String selectPersonLike(final String id, final String firstName, final String lastName) {return new SQL() {{SELECT("P.ID, P.USERNAME, P.PASSWORD, P.FIRST_NAME, P.LAST_NAME");FROM("PERSON P");if (id != null) {WHERE("P.ID like #{id}");}if (firstName != null) {WHERE("P.FIRST_NAME like #{firstName}");}if (lastName != null) {WHERE("P.LAST_NAME like #{lastName}");}ORDER_BY("P.LAST_NAME");}}.toString();
}public String deletePersonSql() {return new SQL() {{DELETE_FROM("PERSON");WHERE("ID = #{id}");}}.toString();
}public String insertPersonSql() {return new SQL() {{INSERT_INTO("PERSON");VALUES("ID, FIRST_NAME", "#{id}, #{firstName}");VALUES("LAST_NAME", "#{lastName}");}}.toString();
}public String updatePersonSql() {return new SQL() {{UPDATE("PERSON");SET("FIRST_NAME = #{firstName}");WHERE("ID = #{id}");}}.toString();
}