可用性和性能考虑

虽然通过视图、子查询可实现与表表达式等同的功能，但表表达式简化查询的公式，使得语法更加灵活和直观，并支持 SQL 的 ANSI/ISO 标准。

查询优化器不具体化 FROM 子句指定的简单的表表达式。与使用 GBase 8s 扩展 TABLE (MULTISET ( SELECT ... )) 语法来在 FROM 子句中指定等同的派生的表的查询的性能比起来，在 FROM 子句中对表表达式使用 ANSI/ISO 语法的查询的性能至少一样好，设置如 UNION、INTERSECT 或 MINUS 操作符这样的操作符，或将 ORDER BY 规范作为复合的表表达式实现，可造成比简单的表表达式更大的开销。请使用 SET EXPLAIN 语句来检测查询计划和表表达式的预计成本。

下列是有效的表表达式的示例：

SELECT * FROM (SELECT * FROM t);

SELECT * FROM (SELECT * FROM t) AS s;

SELECT * FROM (SELECT * FROM t) AS s WHERE t.a = s.b;

SELECT * FROM (SELECT * FROM t) AS s, (SELECT * FROM u) AS v WHERE s.a = v.b;

SELECT * FROM (SELECT SKIP 2 col1 FROM tab1 WHERE col1 > 50 ORDER BY col1  DESC);

SELECT * FROM (SELECT col1,col3 FROM tab1 
   WHERE col1 < 50 GROUP BY col1,col3 ORDER BY col3 ) vtab(vcol0,vcol1);

SELECT * FROM (SELECT * FROM t WHERE t.a = 1) AS s, 
OUTER
(SELECT * FROM u WHERE u.b = 2 GROUP BY 1) AS v WHERE s.a = v.b;

SELECT * FROM (SELECT a AS colA FROM t WHERE t.a = 1) AS s, 
OUTER
(SELECT b AS colB FROM u WHERE u.b = 2 GROUP BY 1) AS v
   WHERE s.colA = v.colB;

CREATE VIEW vu AS SELECT * FROM (SELECT * FROM t);

SELECT * FROM ((SELECT * FROM t) AS r) AS s;