Conditional Expressions
CASE
The standard SQL CASE
expression has two forms.
The “simple” form searches each value
expression from left to right
until it finds one that equals expression
:
CASE expression
WHEN value THEN result
[ WHEN ... ]
[ ELSE result ]
END
The result
for the matching value
is returned.
If no match is found, the result
from the ELSE
clause is
returned if it exists, otherwise null is returned. Example:
SELECT a,
CASE a
WHEN 1 THEN 'one'
WHEN 2 THEN 'two'
ELSE 'many'
END
The “searched” form evaluates each boolean condition
from left
to right until one is true and returns the matching result
:
CASE
WHEN condition THEN result
[ WHEN ... ]
[ ELSE result ]
END
If no conditions are true, the result
from the ELSE
clause is
returned if it exists, otherwise null is returned. Example:
SELECT a, b,
CASE
WHEN a = 1 THEN 'aaa'
WHEN b = 2 THEN 'bbb'
ELSE 'ccc'
END
SQL routines can use CASE
statements that use a slightly
different syntax from the CASE expressions. Specifically note the requirements
for terminating each clause with a semicolon ;
and the usage of END CASE
.
IF
The IF
expression has two forms, one supplying only a
true_value
and the other supplying both a true_value
and a
false_value
:
if(condition, true_value)
Evaluates and returns true_value
if condition
is true,
otherwise null is returned and true_value
is not evaluated.
if(condition, true_value, false_value)
Evaluates and returns true_value
if condition
is true,
otherwise evaluates and returns false_value
.
The following IF
and CASE
expressions are equivalent:
SELECT
orderkey,
totalprice,
IF(totalprice >= 150000, 'High Value', 'Low Value')
FROM tpch.sf1.orders;
SELECT
orderkey,
totalprice,
CASE
WHEN totalprice >= 150000 THEN 'High Value'
ELSE 'Low Value'
END
FROM tpch.sf1.orders;
SQL routines can use IF
statements that use a slightly
different syntax from IF
expressions. Specifically note the requirement
for terminating each clause with a semicolon ;
and the usage of END IF
.
COALESCE
coalesce(value1, value2[, ...])
Returns the first non-null value
in the argument list.
Like a CASE
expression, arguments are only evaluated if necessary.
NULLIF
nullif(value1, value2)
Returns null if value1
equals value2
, otherwise returns value1
.
TRY
try(expression)
Evaluate an expression and handle certain types of errors by returning
NULL
.
In cases where it is preferable that queries produce NULL
or default values
instead of failing when corrupt or invalid data is encountered, the TRY
function may be useful. To specify default values, the TRY
function can be
used in conjunction with the COALESCE
function.
The following errors are handled by TRY
:
- Division by zero
- Invalid cast or function argument
- Numeric value out of range
Examples
Source table with some invalid data:
SELECT * FROM shipping;
origin_state | origin_zip | packages | total_cost
--------------+------------+----------+------------
California | 94131 | 25 | 100
California | P332a | 5 | 72
California | 94025 | 0 | 155
New Jersey | 08544 | 225 | 490
(4 rows)
Query failure without TRY
:
SELECT CAST(origin_zip AS BIGINT) FROM shipping;
Query failed: Cannot cast 'P332a' to BIGINT
NULL
values with TRY
:
SELECT TRY(CAST(origin_zip AS BIGINT)) FROM shipping;
origin_zip
------------
94131
NULL
94025
08544
(4 rows)
Query failure without TRY
:
SELECT total_cost / packages AS per_package FROM shipping;
Query failed: Division by zero
Default values with TRY
and COALESCE
:
SELECT COALESCE(TRY(total_cost / packages), 0) AS per_package FROM shipping;
per_package
-------------
4
14
0
19
(4 rows)