6.3.2 Nested Rules

Rule bodies can contain other rules to combine conditions, collections and generated arguments. The outer rule first decides whether to use its body, then parses inner rules. text and iftext emit literal content without executing inner rules.

Conditions and collections

This query expands an IN clause when the list is nonempty. Use the people table from SQL Execution.

hint FRAGMENT_SQL_OPEN_PACKAGE = 'off';
hint FRAGMENT_SQL_COLUMN_CASE = 'lower';
var find = @@selectSql(ids)<%
    SELECT id, name FROM people WHERE enabled = 1
    @{ifand, ids != null and ids.size() > 0, id IN @{in, #{ids}}}
    ORDER BY id
%>;
return find([1, 2]);

It produces WHERE enabled = 1 AND id IN (?, ?), with [1, 2]. ifand checks the list, in expands placeholders, then ifand adds AND. An empty list leaves only enabled = 1; handle empty input before querying when it should return no rows.

Alternatively, place the complete condition inside in and let and manage its connector:

SELECT * FROM people WHERE enabled = 1
@{and, @{in, id IN #{ids}}}

An empty list makes the inner SQL empty and the outer rule emits nothing. With @{and, id IN @{in, #{ids}}}, an all-null list also omits the condition because all generated arguments are null. Use ifand for explicit control.

Grouped conditions

Connector rules preserve SQL AND/OR precedence. Parentheses group optional conditions:

hint FRAGMENT_SQL_OPEN_PACKAGE = 'off';
hint FRAGMENT_SQL_COLUMN_CASE = 'lower';
var find = @@selectSql(name, minAge)<%
    SELECT id, name FROM people WHERE enabled = 1
    @{if, name != null or minAge != null,
        AND (1 = 0
            @{or, name = #{name}}
            @{or, age >= #{minAge}}
        )
    }
    ORDER BY id
%>;
return find('Alice', 30);

This produces WHERE enabled = 1 AND (1 = 0 OR name = ? OR age >= ?), with ["Alice", 30]. When both arguments are null, the outer if skips the whole group, avoiding empty parentheses.

Branches and collections

case runs only the first selected branch. An explicit empty-list branch guarantees an empty result:

hint FRAGMENT_SQL_OPEN_PACKAGE = 'off';
hint FRAGMENT_SQL_COLUMN_CASE = 'lower';
var find = @@selectSql(ids)<%
    SELECT id, name FROM people WHERE enabled = 1
    @{case, ,
        @{when, ids != null and ids.size() > 0,
            @{in, AND id IN #{ids}}
        }
        @{else, AND 1 = 0}
    }
    ORDER BY id
%>;
return find([]);

An empty list generates AND 1 = 0 and returns []. Input [1] generates AND id IN (?), binding 1. Unselected branches do not generate parameters or look up macros.

Loops and separators

Each pairs template can contain conditional rules. Here the iteration index controls UNION ALL:

hint FRAGMENT_SQL_OPEN_PACKAGE = 'off';
hint FRAGMENT_SQL_COLUMN_CASE = 'lower';
var rows = @@selectSql(names)<%
    @{pairs, #{names},
        @{iftext, i > 0, UNION ALL}
        SELECT CAST(#{v} AS VARCHAR) AS name
    }
%>;
return rows(['Alice', 'Bob']);

CAST supplies an explicit VARCHAR type for parameters in the UNION query.

Generated SQL is SELECT CAST(? AS VARCHAR) AS name UNION ALL SELECT CAST(? AS VARCHAR) AS name, returning [{"name":"Alice"},{"name":"Bob"}]. Empty input generates no SQL, so skip this call in the script for an empty list.

Generated arguments and null checks

SELECT * FROM payload_log WHERE 1 = 1
@{ifand, content != null, digest = @{md5, #{content}}}

With content = "abc", MD5 generates a digest argument and the outer rule emits AND digest = ?. A null content fails the outer condition, so MD5 is not evaluated.

With @{and, digest = @{md5, #{content}}}, MD5 first converts null to the empty-string digest. The outer and sees a non-null digest and retains the condition. Use if/ifand to check the original input.

Boundaries

• Rules inside SQL strings or comments are not executed. For example, '@{uuid32}' is a string literal.
• text and iftext do not parse placeholders or inner rules. Use if when parsing is required.
• Prefer named parameters in dynamic rules so nested conditions and branches can share them clearly.
• Rules control their own content. They cannot remove dangling AND keywords, commas or parentheses outside that content. Put an optional connector and its SQL in the same conditional branch.