Structured Query Language/SELECT: Case Expression
Sometimes it's necessary to translate stored values (or values to be stored) from one representation to another. Suppose there is a column status with legal values from 0 to 9, but the end-users should receive strings that explain the meaning of the numeric values in short, eg.: 'ordered', 'delivered', 'back delivery', 'out of stock', ... . The recommended way to do this is a separate table where the numeric values maps to the explanatory strings. Notwithstanding this, application developers may favor a solution within an application server.
The CASE expression, which is shown on this page, is a technique for solving the described situation as part of a SELECT, INSERT or UPDATE command as well as solving additional problems. As part of the language, it's a powerful term that can be applied at plenty of places within SQL commands. On this page, we focus on its use together with the SELECT command. The strategy and syntax for CASE within INSERT and UPDATE are equivalent and are presented over there. In comparison with the recommended technique of a separate table for the translation, the CASE expression is much more flexible (which is not an advantage in all cases).
Two Examples[edit | edit source]
-- Technical term: "simple case" -- Select id, contact_type in a translated version and contact_value SELECT id, CASE contact_type WHEN 'fixed line' THEN 'Phone' WHEN 'mobile' THEN 'Phone' ELSE 'Not a telephone number' END AS 'contact_type', contact_value FROM contact;
The CASE expression is introduced with its keyword CASE and runs up to the END keyword. In this first example, it specifies a column name and a series of WHEN/THEN clauses with an optional ELSE clause. The WHEN/THEN clauses are compared and evaluated against the values of the named column, one after the other. If none of them hits, the ELSE clause applies. If there is no ELSE clause and non of the WHEN/THEN clauses hit, the NULL special marker will be applied.
The comparison between the values of the column and the fixed values within the WHEN/THEN clause is done solely by "=" (equals). This is a good starting point, but real applications need more than that. Therefore there is a variant of the CASE.
-- Technical term: "searched case" -- Select persons name, weight and a denomination of the weight SELECT firstname, lastname, weight, CASE WHEN (weight IS NULL OR weight = 0) THEN 'weight is unknown' WHEN weight < 40 THEN 'lightweight' WHEN weight BETWEEN 40 AND 85 THEN 'medium' ELSE 'heavyweight' END AS 'weight' FROM person;
The crucial point is the direct succession of the two keywords CASE and WHEN. There is no column name between them. In this variant, there must be a complete expression, which evaluates to one of the 3-value-logic terms true, false or unknown, between each WHEN and THEN. Now it is possible to use all the comparisons and boolean operators as they are known by the WHERE clause. It is even possible to compare different columns or function calls with each other.
Syntax[edit | edit source]
There are the two variants simple case and searched case.
-- "simple case" performs successive comparisons using the equal operator: <column_name> = <expression_x> CASE <column_name> WHEN <expression_1> THEN <result_1> WHEN <expression_2> THEN <result_2> ... ELSE <default_result> -- optional END -- "searched case" is recognised by 'nothing' between CASE and first WHEN CASE WHEN <condition_1> THEN <result_1> WHEN <condition_2> THEN <result_2> ... ELSE <default_result> -- optional END
The simple case is limited to one column and the use of the equal operator, whereas the searched case may evaluate arbitrary columns of the (intermediate) result with arbitrary operators, functions or predicates.
Typical Use Cases[edit | edit source]
The use of CASE expressions is not limited to projections (the column list between SELECT and FROM). As the clause evaluates to a value, it can be applied as a substitution for values at several places within SQL commands. In the following, we offer some examples.
ORDER BY clause
Sort contact values in the order: all fixed lines, all mobile phones, all emails, all icq's. Within each group sort over the contact values.
SELECT * FROM contact ORDER BY -- a "simple case" construct as substitution for a column name CASE contact_type WHEN 'fixed line' THEN 0 WHEN 'mobile' THEN 1 WHEN 'email' THEN 2 WHEN 'icq' THEN 3 ELSE 4 END, contact_value;
In the next example, persons are ordered by weight classes, within the classes by their name.
-- order by weight classes SELECT firstname, lastname, weight, CASE WHEN (weight IS NULL OR weight = 0) THEN 'weight is unknown' WHEN weight < 40 THEN 'lightweight' WHEN weight BETWEEN 40 AND 85 THEN 'medium' ELSE 'heavyweight' END FROM person ORDER BY -- a "searched case" construct with IS NULL, BETWEEN and 'less than'. CASE WHEN (weight IS NULL OR weight = 0) THEN 0 WHEN weight < 40 THEN 1 WHEN weight BETWEEN 40 AND 85 THEN 2 ELSE 3 END, lastname, firstname;
Within the WHERE clauses, there may occur fixed values or column names. CASE expressions can be used as a substitution for them. In the example, persons receive a discount on their weight depending on their place of birth (consider it as a theoretical example). Thus Mr. Goldstein, with its 95 kg, counts only with 76 kg and is not part of the result set.
SELECT * FROM person WHERE CASE -- Modify weight depending on place of birth. WHEN place_of_birth = 'Dallas' THEN weight * 0.8 WHEN place_of_birth = 'Richland' THEN weight * 0.9 ELSE weight END > 80 OR weight < 20; -- any other condition
Exercises[edit | edit source]
Show firstname, lastname and the gender of all persons. Consider Larry, Tom, James, John, Elias, Yorgos, Victor as 'male',
Lisa as 'female' and all others as 'unknown gender'. Use a simple case expression.
SELECT firstname, lastname, CASE firstname WHEN 'Larry' THEN 'male' WHEN 'Tom' THEN 'male' WHEN 'James' THEN 'male' WHEN 'John' THEN 'male' WHEN 'Elias' THEN 'male' WHEN 'Yorgos' THEN 'male' WHEN 'Victor' THEN 'male' WHEN 'Lisa' THEN 'female' ELSE 'unknown gender' END FROM person;
Use a searched case expression to solve the previous question.
SELECT firstname, lastname, CASE WHEN firstname in ('Larry', 'Tom', 'James', 'John', 'Elias', 'Yorgos', 'Victor') THEN 'male' WHEN firstname = 'Lisa' THEN 'female' ELSE 'unknown gender' END FROM person;
Show firstname, lastname and a classification of all persons. Classify persons according to the length of their firstname. Call the class 'short name' if character_length(firstname) < 4, 'medium length' if < 6, 'long name' else.
-- Hint: Some implementations use a different function name: length() or len(). SELECT firstname, lastname, CASE WHEN CHARACTER_LENGTH(firstname) < 4 THEN 'short name' WHEN CHARACTER_LENGTH(firstname) < 6 THEN 'medium length' ELSE 'long name' END FROM person;
-- Hint: Some implementations use a different function name: length() or len(). SELECT SUM(CASE WHEN CHARACTER_LENGTH(firstname) < 4 THEN 1 ELSE 0 END) as short_names, SUM(CASE WHEN CHARACTER_LENGTH(firstname) between 4 and 5 THEN 1 ELSE 0 END) as medium, SUM(CASE WHEN CHARACTER_LENGTH(firstname) > 5 THEN 1 ELSE 0 END) as long_names FROM person;