The Problem

One of the nastiest things a Java developer will ever have to do is embed SQL in Java code. Usually this is done because the SQL has to be dynamically generated - otherwise you could externalize it in a file or a stored proc. As you've already seen, MyBatis has a powerful answer for dynamic SQL generation in its XML mapping features. However, sometimes it becomes necessary to build a SQL statement string inside of Java code. In that case, MyBatis has one more feature to help you out, before reducing yourself to the typical mess of plus signs, quotes, newlines, formatting problems and nested conditionals to deal with extra commas or AND conjunctions. Indeed, dynamically generating SQL code in Java can be a real nightmare. For example:

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";

The Solution

MyBatis 3 offers a convenient utility class to help with the problem. With the SQL class, you simply create an instance that lets you call methods against it to build a SQL statement one step at a time. The example problem above would look like this when rewritten with the SQL class:

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();
}

What is so special about that example? Well, if you look closely, it doesn't have to worry about accidentally duplicating “AND” keywords, or choosing between “WHERE” and “AND” or none at all. The SQL class takes care of understanding where “WHERE” needs to go, where an “AND” should be used and all of the String concatenation.

The SQL Class

Here are some examples:

// 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();
}

NOTE It is important to note that SQL class writes LIMIT, OFFSET, OFFSET n ROWS and FETCH FIRST n ROWS ONLY clauses into the generated statement as is. In other words, the library does not attempt to normalize those values for databases that don’t support these clauses directly. Therefore, it is very important for users to understand whether or not the target database supports these clauses. If the target database does not support these clauses, then it is likely that using this support will create SQL that has runtime errors.

Since version 3.4.2, you can use variable-length arguments as follows:

public String selectPersonSql() {
  return new SQL()
    .SELECT("P.ID", "A.USERNAME", "A.PASSWORD", "P.FULL_NAME", "D.DEPARTMENT_NAME", "C.COMPANY_NAME")
    .FROM("PERSON P", "ACCOUNT A")
    .INNER_JOIN("DEPARTMENT D on D.ID = P.DEPARTMENT_ID", "COMPANY C on D.COMPANY_ID = C.ID")
    .WHERE("P.ID = A.ID", "P.FULL_NAME like #{name}")
    .ORDER_BY("P.ID", "P.FULL_NAME")
    .toString();
}

public String insertPersonSql() {
  return new SQL()
    .INSERT_INTO("PERSON")
    .INTO_COLUMNS("ID", "FULL_NAME")
    .INTO_VALUES("#{id}", "#{fullName}")
    .toString();
}

public String updatePersonSql() {
  return new SQL()
    .UPDATE("PERSON")
    .SET("FULL_NAME = #{fullName}", "DATE_OF_BIRTH = #{dateOfBirth}")
    .WHERE("ID = #{id}")
    .toString();
}

Since version 3.5.2, you can create insert statement for bulk insert as follow:

public String insertPersonsSql() {
  // INSERT INTO PERSON (ID, FULL_NAME)
  //     VALUES (#{mainPerson.id}, #{mainPerson.fullName}) , (#{subPerson.id}, #{subPerson.fullName})
  return new SQL()
    .INSERT_INTO("PERSON")
    .INTO_COLUMNS("ID", "FULL_NAME")
    .INTO_VALUES("#{mainPerson.id}", "#{mainPerson.fullName}")
    .ADD_ROW()
    .INTO_VALUES("#{subPerson.id}", "#{subPerson.fullName}")
    .toString();
}

Since version 3.5.2, you can create select statement for limiting search result rows clause as follow:

public String selectPersonsWithOffsetLimitSql() {
  // SELECT id, name FROM PERSON
  //     LIMIT #{limit} OFFSET #{offset}
  return new SQL()
    .SELECT("id", "name")
    .FROM("PERSON")
    .LIMIT("#{limit}")
    .OFFSET("#{offset}")
    .toString();
}

public String selectPersonsWithFetchFirstSql() {
  // SELECT id, name FROM PERSON
  //     OFFSET #{offset} ROWS FETCH FIRST #{limit} ROWS ONLY
  return new SQL()
    .SELECT("id", "name")
    .FROM("PERSON")
    .OFFSET_ROWS("#{offset}")
    .FETCH_FIRST_ROWS_ONLY("#{limit}")
    .toString();
}

SqlBuilder and SelectBuilder (DEPRECATED)

Before version 3.2 we took a bit of a different approach, by utilizing a ThreadLocal variable to mask some of the language limitations that make Java DSLs a bit cumbersome. However, this approach is now deprecated, as modern frameworks have warmed people to the idea of using builder-type patterns and anonymous inner classes for such things. Therefore the SelectBuilder and SqlBuilder classes have been deprecated.

The following methods apply to only the deprecated SqlBuilder and SelectBuilder classes.

The SelectBuilder and SqlBuilder classes are not magical, but it's important to know how they work. SelectBuilder and SqlBuilder use a combination of Static Imports and a ThreadLocal variable to enable a clean syntax that can be easily interlaced with conditionals. To use them, you statically import the methods from the classes like this (one or the other, not both):

import static org.apache.ibatis.jdbc.SelectBuilder.*;

import static org.apache.ibatis.jdbc.SqlBuilder.*;

This allows us to create methods like these:

/* DEPRECATED */
public String selectBlogsSql() {
  BEGIN(); // Clears ThreadLocal variable
  SELECT("*");
  FROM("BLOG");
  return SQL();
}

/* DEPRECATED */
private String selectPersonSql() {
  BEGIN(); // Clears ThreadLocal variable
  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");
  return SQL();
}