Subquery in where clause
Subquery in simple comparison condition
Syntax:
Description:
The following rules are applied to the simple comparison condition with subquery in WHERE clause:
Subquery should returns single row.
If left hand side is expr, than subquery must return single row with the only value that type compatible withexpr.
If left hand side is expr_list, than subquery must return single row with the list of values that corresponds in number and type withexpr_list. In this case comparison operator returns TRUE if every value inexpr_listis equal (in case =) or not equal (in case !=, ^=, <>) to the every value returned by query.
Examples:
1. Select departments that are located in the same building as informatics faculty:
SELECT Name
FROM DEPARTMENT
WHERE Building = (SELECT Building
FROM FACULTY
WHERE Name = 'informatics');
2. Select faculties that have funds less than fund of CAD department
SELECT Name
FROM FACULTY
WHERE Fund <(SELECT Fund
FROM DEPARTMENT
WHERE Name = 'CAD');
3. Select teachers that salary + commission is more than 100 of the half of salary + commission if Bill:
SELECT Name
FROM TEACHER
WHERE Salary + Commission + 100 > (SELECT (Salary + Commission) / 2
FROM TEACHER
WHERE Name = 'Bill');
4. Select teachers that have the same department No and post as Bill:
SELECT Name
FROM TEACHER
WHERE (DepNo, Post)= (SELECT (DepNo, Post
FROM TEACHER
WHERE Name = 'Bill');
Subquery in group comparison condition
Syntax:
Description:
The following rules are applied to the group comparison condition with subquery in WHERE clause:
Subquery may return zero or more rows.
If left hand side is expr, than subquery must return rows with the only value that type compatible withexpr.
If left hand side is expr_list, than subquery must return rows with the list of values that corresponds in number and type withexpr_list.
ANY and SOME are equivalent and compares a value to each value in a list of rowsreturned by a query. Query may return zero or more rows. Evaluates toTRUE if at least one row returned by query is in relation (corresponding comparison operator) with value (list of values) defined by the first operand, otherwise it evaluates FALSE. If query does not returns any row it evaluates to FALSE
ALL compares a value (list of values)to every value (list of values)in a list of rowsreturned by a query. It Evaluates toTRUE if ALL rows returned by query is in relation (corresponding comparison operator) with value (list of values) defined by the first operand, otherwise it evaluates FALSE. If query does not returns any row it evaluates to TRUE
Examples:
1. Display departments with fund that is more that fund of at least one faculty:
SELECT Name
FROM DEPARTMENT
WHERE Fund > ANY (SELECT Fund FROM FACULTY);
ANY and aggregate functions. Pay attention, that operator <ANY is equivalent the following statement: “the left value is less than maximum of right values”, and operator >ANY is equivalent the following statement: “the left value is more than minimum of right values”. That is why these ANY operators may be expressed with the help of MAX and MIN functions in subquery.
2. Display departments with fund that is more that fund of at least one faculty:
SELECT Name
FROM DEPARTMENT
WHERE Fund > ANY (SELECT Fund FROM FACULTY);
SELECT Name
FROM DEPARTMENT
WHERE Fund > (SELECT MIN(Fund) FROM FACULTY);
3. Display groups that have ratings more than ratings of all groups of the fifth course of “DBMS” department:
SELECT Num
FROM GROUP
WHERE Rating >ALL (SELECT Rating
FROM GROUP, DEPARTMENT
WHERE GROUP.DepNo = DEPARTMENT.DepNo AND
DEPARTMENT.Name = 'DBMS' AND GROUP.Course = 5);