Merge pull request #8373 from soyeric128/scalar-subqueries

docs: scalar subqueries

docs/doc/30-reference/30-sql/30-query-operators/operators-subquery.md

@@ -4,11 +4,47 @@ description:
   A subquery is a query nested within another query.
 ---
 
-This topic provides reference information about the subquery operators supported in Databend.
+A subquery is a query nested within another one. Databend supports the following subquery types:
 
-A subquery is a query nested within another query.
+- [Scalar Subquery](#scalar-subquery)
+- [EXISTS / NOT EXISTS](#exists--not-exists)
 
-## [ NOT ] EXISTS
+## Scalar Subqueries
+
+A scalar subquery selects only one column or expression and returns only one row at most. A SQL query can have scalar subqueries in any places where a column or expression is expected.
+
+- If a scalar subquery returns 0 rows, Databend will use NULL as the subquery output.
+- If a scalar subquery returns more than one row, Databend will throw an error.
+
+### Example
+
+```sql
+CREATE TABLE t1 (a int);
+CREATE TABLE t2 (a int);
+
+INSERT INTO t1 VALUES (1);
+INSERT INTO t1 VALUES (2);
+INSERT INTO t1 VALUES (3);
+
+INSERT INTO t2 VALUES (3);
+INSERT INTO t2 VALUES (4);
+INSERT INTO t2 VALUES (5);
+
+SELECT * 
+FROM   t1 
+WHERE  t1.a < (SELECT Min(t2.a) 
+               FROM   t2); 
+
+--
++--------+
+|      a |
++--------+
+|      1 |
+|      2 |
++--------+
+```
+
+## EXISTS / NOT EXISTS
 
 An EXISTS subquery is a boolean expression that can appear in a WHERE clause:
 * An EXISTS expression evaluates to TRUE if any rows are produced by the subquery.