Subquery Expressions

Subquery expressions include the following types:

EXISTS/NOT EXISTS

Figure 1 shows the syntax of an EXISTS/NOT EXISTS expression.

Figure 1 EXISTS/NOT EXISTS::=
Click to enlarge

The parameter of an EXISTS expression is an arbitrary SELECT statement, or subquery. The subquery is evaluated to determine whether it returns any rows. If it returns at least one row, the result of EXISTS is "true". If the subquery returns no rows, the result of EXISTS is "false".

The subquery will generally only be executed long enough to determine whether at least one row is returned, not all the way to completion.

For example:

     
      
        
        
          openGauss=# SELECT sr_reason_sk,sr_customer_sk FROM tpcds.store_returns WHERE EXISTS (SELECT d_dom FROM tpcds.date_dim WHERE d_dom = store_returns.sr_reason_sk and sr_customer_sk <10);
sr_reason_sk | sr_customer_sk 
--------------+----------------
           13 |              2
           22 |              5
           17 |              7
           25 |              7
            3 |              7
           31 |              5
            7 |              7
           14 |              6
           20 |              4
            5 |              6
           10 |              3
            1 |              5
           15 |              2
            4 |              1
           26 |              3
(15 rows)

         

       

     
    

IN/NOT IN

Figure 2 shows the syntax of an IN/NOT IN expression.

Figure 2 IN/NOT IN::=
Click to enlarge

The right-hand side is a parenthesized subquery, which must return exactly one column. The left-hand expression is evaluated and compared to each row of the subquery result. The result of IN is "true" if any equal subquery row is found. The result is "false" if no equal row is found (including the case where the subquery returns no rows).

This is in accordance with SQL normal rules for Boolean combinations of null values. If the columns corresponding to two rows equal and are not empty, the two rows are equal to each other. If any columns corresponding to the two rows do not equal and are not empty, the two rows are not equal to each other. Otherwise, the result is NULL. If there are no equal right-hand values and at least one right-hand row yields null, the result of IN will be null, not false.

For example:

     
      
        
        
          openGauss=# SELECT sr_reason_sk,sr_customer_sk  FROM tpcds.store_returns WHERE sr_customer_sk IN (SELECT d_dom FROM tpcds.date_dim WHERE d_dom < 10);
sr_reason_sk | sr_customer_sk 
--------------+----------------
           10 |              3
           26 |              3
           22 |              5
           31 |              5
            1 |              5
           32 |              5
           32 |              5
            4 |              1
           15 |              2
           13 |              2
           33 |              4
           20 |              4
           33 |              8
            5 |              6
           14 |              6
           17 |              7
            3 |              7
           25 |              7
            7 |              7
(19 rows)

         

       

     
    

ANY/SOME

Figure 3 shows the syntax of an ANY/SOME expression.

Figure 3 any/some::=
Click to enlarge

The right-hand side is a parenthesized subquery, which must return exactly one column. The left-hand expression is evaluated and compared to each row of the subquery result using the given operator, which must yield a Boolean result. The result of ANY is "true" if any true result is obtained. The result is "false" if no true result is found (including the case where the subquery returns no rows). SOME is a synonym of ANY. IN can be equivalently replaced with ANY.

For example:

     
      
        
        
          openGauss=# SELECT sr_reason_sk,sr_customer_sk  FROM tpcds.store_returns WHERE sr_customer_sk < ANY (SELECT d_dom FROM tpcds.date_dim WHERE d_dom < 10);
sr_reason_sk | sr_customer_sk 
--------------+----------------
           26 |              3
           17 |              7
           32 |              5
           32 |              5
           13 |              2
           31 |              5
           25 |              7
            5 |              6
            7 |              7
           10 |              3
            1 |              5
           14 |              6
            4 |              1
            3 |              7
           22 |              5
           33 |              4
           20 |              4
           33 |              8
           15 |              2
(19 rows)

         

       

     
    

ALL

Figure 4 shows the syntax of an ALL expression.

Figure 4 all::=
Click to enlarge

The right-hand side is a parenthesized subquery, which must return exactly one column. The left-hand expression is evaluated and compared to each row of the subquery result using the given operator, which must yield a Boolean result. The result of ALL is "true" if all rows yield true (including the case where the subquery returns no rows). The result is "false" if any false result is found.

For example:

     
          openGauss=# SELECT sr_reason_sk,sr_customer_sk  FROM tpcds.store_returns WHERE sr_customer_sk < all(SELECT d_dom FROMOM tpcds.date_dim WHERE d_dom < 10);
 sr_reason_sk | sr_customer_sk 
--------------+----------------
(0 rows)