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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。此时，MyBatis有另外一个特性可以帮到你，在减少典型的加号,引号,新行,格式化问题和嵌入条件来处理多余的逗号或 AND 连接词之前。事实上，在Java代码中来动态生成SQL代码就是一场噩梦。例如：
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";
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();
}
该例中有什么特殊之处？当你仔细看时，那不用担心偶然间重复出现的"AND"关键字，或者在"WHERE"和"AND"之间的选择，抑或什么都不选。该SQL类非常注意"WHERE"应该出现在何处，哪里又应该使用"AND"，还有所有的字符串链接。
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(