Expressions（2）

Subquery expressions include the following types:

· EXISTS/NOT EXISTS

Figure 8 shows the syntax of an EXISTS/NOT EXISTS expression.

Figure 8 EXISTS/NOT EXISTS::=

The parameter of an EXISTS expression is an arbitrary SELECT statement, or a subquery. The subquery is computed to determine whether it returns any rows. If it returns at least one row, the result of EXISTS is true. If it 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.

Example:

Note: In the following query statement, tpcds indicates the schema name.

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);

· IN/NOT IN

Figure 9 shows the syntax of an IN/NOT IN expression.

Figure 9 IN/NOT IN::=

The right-side parenthesized subquery returns only one column. The left-side expression is calculated 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 null, the two rows are equal to each other. If any columns corresponding to the two rows do not equal and are not null, the two rows are not equal to each other. Otherwise, the result is NULL. If the result of each row does not equal and at least one row yields NULL, the result of IN will be NULL.

Example:

Note: In the following query statement, tpcds indicates the schema name
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);

· ANY/SOME

Figure 10 shows the syntax of an ANY/SOME expression.

Figure 10 any/some::=

The right-side parenthesized subquery returns only one column. The left-side expression is calculated and compared to each row of the subquery result by using a 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 by ANY.

Example:

Note: In the following query statement, tpcds indicates the schema name.
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);

· ALL

Figure 11 shows the syntax of an ALL expression.

Figure 11 all::=

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