SQL Conditional Expressions
Conditional Expressions Summary Table
| Function | Description |
|---|---|
Case operator |
|
Return the first non- |
|
Returns the largest value from a list of any number of expressions |
|
|
|
Check whether a value is within a set of values |
|
Returns the smallest value from a list of any number of expressions |
|
Return |
CASE
There are two key uses for the CASE expression:
-
A more generic version where every condition should be a boolean expression evaluated on true or false, and the first evaluated to true determines the result.
-
Or CaseSearch where a value expression is employed to determine the branch used.
Description
CASE clauses can be used wherever an expression is valid. condition is an expression that returns a boolean result.
If the result is:
-
True, then the value of the
CASEexpression is the result that follows the condition. -
False, any subsequent
WHENclauses are searched in the same manner.
If no WHEN condition is true then the value of the case expression is the result in the ELSE clause.
If the ELSE clause is omitted and no condition matches, the result is NULL.
NuoDB enforces the same type, precision and scale for all the branches of the CASE expression. In situations where the branches do not have the same type, all the branches are coerced to a computed common type. The coercions are enforced during constant folding, which means that when case coercions are applied, a query might fail with a coercion error even if the failing branch would not have been part of the result.
For the generic version of CASE, each predicate in the WHEN clause is type checked in isolation and coercions are applied following the comparison operators model. With the simplified version of the CASE expression, the type of search condition and the types of the WHEN clauses have to be coercible to a common type.
Example
CREATE TABLE tst_table (P int);
INSERT INTO tst_table VALUES (1),(2),(NULL),(20);
SELECT CASE WHEN P = 1 THEN 'one'
WHEN P is null THEN 'null'
WHEN P > 10 THEN 'big'
ELSE 'unknown' END
from tst_table;
[CASE]
-------
one
unknown
null
big
SELECT CASE P
WHEN 1 THEN 'one'
WHEN 2 THEN 'two'
ELSE 'big' END
from tst_table;
[CASE]
-------
one
two
big
bigCOALESCE
Description
Returns the first of its arguments that is not null. Null is returned only if all arguments are null. It is often used to substitute a default value for null values when data is retrieved for display.
Like a CASE expression, COALESCE will not evaluate arguments that are not needed to determine the result (that is, arguments to the right of the first non-null argument are not evaluated).
GREATEST
Description
Returns the largest value from a list of two or more expressions.
If the list of expressions is not all of the same data type, expressions in the list will be converted to the common type computed based on the types of the arguments. An error is returned if a common type does not exist or if not all expressions are convertible to the common type.
Returns NULL if any expression in the list is NULL.
Example
SELECT GREATEST('abc', 'ABC') FROM DUAL;
[GREATEST]
-----------
abc
SELECT GREATEST('abc', 'ABC', NULL) FROM DUAL;
[GREATEST]
-----------
<null>
/* Count teams that have more LOSSES than WINS or TIES */
SELECT COUNT(*) FROM hockey.teams
WHERE GREATEST(wins, losses, ties) = losses;
[COUNT]
--------
685
SELECT GREATEST(1,2,'abc') FROM DUAL;
Error 22000: unable to convert string "abc" into type "number"IFNULL
Description
If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.
The returned type of IFNULL is the common type of the two arguments.
Both arguments must be coercible to the common type.
Example
SELECT IFNULL(1,0) FROM dual;
[IFNULL]
---------
1
SELECT IFNULL(NULL,10) FROM dual;
[IFNULL]
---------
10
SELECT 1.0000/10.0000, IFNULL(1.000/null,1.000/10.0000), IFNULL(1.000/null,10.0000), IFNULL(null/10.000,1.000) FROM dual;
[/] [IFNULL] [IFNULL] [IFNULL]
------------ ----------- --------- ---------
.10000000000 .1000000000 10.0000 1.000IN
Syntax
WHERE column IN (x1,x2,x3 [,...] )which is equivalent to:
WHERE ( column=x1 OR column=x2 OR column=x3 [OR ... ] )Description
Checks if the specified column data is a member of the specified list. It can be used with WHERE, CHECK, and creation of views.
The IN operator applies the comparison common type algorithm in two steps, first on the elements in the IN list and then by computing the common type between the search argument and the common type of the IN list. The common type on the list of arguments on the right hand side is computed from left to right by computing the common type between the currently computed common type and the next argument. This makes the IN list not commutative in terms of the types of the arguments.
LEAST
Description
Returns the smallest value from a list of two or more expressions.
If the list of expressions is not all the same data type, NuoDB applies the same type rules as for the GREATEST expression.
Returns NULL if any expression in the list is NULL.
Example
SELECT LEAST('abc', 'ABC') FROM DUAL;
[LEAST]
--------
ABC
SELECT LEAST('abc', 'ABC', NULL) FROM DUAL;
[LEAST]
--------
<null>
/* Count teams that have less WINS than LOSSES or TIES */
SELECT COUNT(*) FROM hockey.teams
WHERE LEAST(wins, losses, ties) = wins;
[COUNT]
--------
20
SELECT LEAST(1,2,'abc') FROM DUAL;
Error 22000: unable to convert string "abc" into type "number"NULLIF
Description
Returns a null value if value1 and value2 are equal; otherwise it returns value1.
An error is returned if value1 and value2 do not have the same type and cannot be coerced to a common type.
Example
CREATE TABLE t (a int, b int);
INSERT INTO t VALUES (null, null), (null, 1), (1, null), (1, 1);
SELECT a, b, nullif (a, b) as nullif FROM t;
A B NULLIF
------ ------ -------
<null> <null> <null>
<null> 1 <null>
1 <null> 1
1 1 <null>
SELECT COALESCE (a, 0) as a, COALESCE (b, 0) as b, COALESCE (nullif (a, b), 0) AS nullif FROM t;
A B NULLIF
-- -- -------
0 0 0
0 1 0
1 0 1
1 1 0