feat: allow SELECT without target expressions for PostgreSQL (#2841)

* feat: allow SELECT without target expressions for PostgreSQL

Resolves #2784

* style: rename new test to camelCase for Codacy

* fix: disambiguate BigQuery AS STRUCT/VALUE qualifier from select tail

Making the select-items group optional for the target-less PostgreSQL
SELECT let JavaCC see K_AS in the follow set of the optional AS
STRUCT/VALUE qualifier block, producing a choice conflict at the block.
Guard it with an explicit lookahead so AS only opens the qualifier when
STRUCT or VALUE comes next.

Author: princejain756

src/main/java/net/sf/jsqlparser/statement/select/PlainSelect.java

@@ -629,10 +629,16 @@ public StringBuilder appendSelectBodyTo(StringBuilder builder) {
 
     private void appendSelectClauseTo(StringBuilder builder) {
         builder.append("SELECT ");
+        int afterSelect = builder.length();
         appendSelectHintsTo(builder);
         appendSelectQualifiersTo(builder);
         appendMySqlSelectModifiersTo(builder);
         builder.append(getStringList(selectItems));
+        // PostgreSQL permits SELECT with no target expressions: keep "SELECT
+        // FROM" instead of doubling the space.
+        if (builder.length() == afterSelect) {
+            builder.deleteCharAt(afterSelect - 1);
+        }
     }
 
     private void appendSelectHintsTo(StringBuilder builder) {

src/main/java/net/sf/jsqlparser/util/deparser/SelectDeParser.java

@@ -379,7 +379,13 @@ private <S> void deparseSelectClause(PlainSelect plainSelect, S context) {
 
         plainSelect.appendMySqlSelectModifiersTo(builder);
 
+        int beforeItems = builder.length();
         deparseSelectItemsClause(plainSelect.getSelectItems());
+        // PostgreSQL permits SELECT with no target expressions: keep "SELECT
+        // FROM" instead of doubling the space.
+        if (builder.length() == beforeItems && builder.charAt(beforeItems - 1) == ' ') {
+            builder.deleteCharAt(beforeItems - 1);
+        }
 
         if (plainSelect.getIntoTables() != null) {
             builder.append(" INTO ");

src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt

@@ -9241,7 +9241,7 @@ PlainSelect PlainSelect() #PlainSelect:
         }
     }
 
-    [
+    [ LOOKAHEAD(<K_AS> (<K_STRUCT> | <K_VALUE>))
         <K_AS>
         (
             <K_STRUCT> { plainSelect.setBigQuerySelectQualifier( PlainSelect.BigQuerySelectQualifier.AS_STRUCT ); }
@@ -9252,7 +9252,14 @@ PlainSelect PlainSelect() #PlainSelect:
 
     [ LOOKAHEAD(2) top = Top() { plainSelect.setTop(top);    } ]
 
-    selectItems=SelectItemsList()
+    (
+        // PostgreSQL allows SELECT with no target expressions.
+        LOOKAHEAD({ Dialect.POSTGRESQL.name().equals(getAsString(Feature.dialect))
+                && getToken(1).kind == K_FROM })
+        { selectItems = new ArrayList<SelectItem<?>>(); }
+        |
+        selectItems=SelectItemsList()
+    )
 
     [ LOOKAHEAD(<K_INTO>)
         (

src/test/java/net/sf/jsqlparser/statement/select/SelectTest.java

@@ -2398,6 +2398,21 @@ public void testBooleanTestAfterPredicate() throws JSQLParserException {
         assertSqlCanBeParsedAndDeparsed("SELECT * FROM t WHERE a IS TRUE IS FALSE");
     }
 
+    @Test
+    public void testSelectWithoutTargetListPostgreSql() throws JSQLParserException {
+        // PostgreSQL permits SELECT with no target expressions
+        Statement stmt = CCJSqlParserUtil.parse(
+                "SELECT FROM t", parser -> parser.withDialect(Dialect.POSTGRESQL));
+        PlainSelect select = (PlainSelect) ((Select) stmt).getSelectBody();
+        assertTrue(select.getSelectItems().isEmpty());
+        assertDeparse(select, "SELECT FROM t");
+
+        // the reported case: empty target list inside a derived table
+        assertSqlCanBeParsedAndDeparsed(
+                "SELECT count(*) FROM (SELECT FROM (VALUES (1, 'a'), (2, 'b')) AS t(id, label)) AS q",
+                false, parser -> parser.withDialect(Dialect.POSTGRESQL));
+    }
+
     @Test
     public void testIsUnknown() throws JSQLParserException {
         String statement = "SELECT col FROM tbl WHERE col IS UNKNOWN";