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451 lines
9.4 KiB
Markdown
451 lines
9.4 KiB
Markdown
---
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mapped_pages:
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- https://www.elastic.co/guide/en/elasticsearch/reference/current/sql-functions-conditional.html
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---
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# Conditional functions and expressions [sql-functions-conditional]
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Functions that return one of their arguments by evaluating in an if-else manner.
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## `CASE` [sql-functions-conditional-case]
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```sql
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CASE WHEN condition THEN result
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[WHEN ...]
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[ELSE default_result]
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END
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```
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**Input**:
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One or multiple *WHEN **condition** THEN **result** clauses are used and the expression can optionally have an *ELSE **default_result** clause. Every **condition** should be a boolean expression.
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**Output**: one of the **result** expressions if the corresponding *WHEN **condition** evaluates to `true` or the **default_result** if all *WHEN **condition** clauses evaluate to `false`. If the optional *ELSE **default_result** clause is missing and all *WHEN **condition** clauses evaluate to `false` then `null` is returned.
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**Description**: The CASE expression is a generic conditional expression which simulates if/else statements of other programming languages If the condition’s result is true, the value of the result expression that follows the condition will be the returned the subsequent when clauses will be skipped and not processed.
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```sql
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SELECT CASE WHEN 1 > 2 THEN 'elastic'
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WHEN 2 <= 3 THEN 'search'
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END AS "case";
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case
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---------------
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search
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```
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```sql
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SELECT CASE WHEN 1 > 2 THEN 'elastic'
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WHEN 2 > 10 THEN 'search'
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END AS "case";
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case
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---------------
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null
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```
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```sql
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SELECT CASE WHEN 1 > 2 THEN 'elastic'
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WHEN 2 > 10 THEN 'search'
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ELSE 'default'
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END AS "case";
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case
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---------------
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default
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```
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As a variant, a case expression can be expressed with a syntax similar to **switch-case** of other programming languages:
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```sql
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CASE expression
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WHEN value1 THEN result1
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[WHEN value2 THEN result2]
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[WHEN ...]
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[ELSE default_result]
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END
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```
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In this case it’s transformed internally to:
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```sql
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CASE WHEN expression = value1 THEN result1
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[WHEN expression = value2 THEN result2]
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[WHEN ...]
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[ELSE default_result]
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END
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```
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```sql
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SELECT CASE 5
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WHEN 1 THEN 'elastic'
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WHEN 2 THEN 'search'
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WHEN 5 THEN 'elasticsearch'
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END AS "case";
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case
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---------------
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elasticsearch
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```
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```sql
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SELECT CASE 5
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WHEN 1 THEN 'elastic'
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WHEN 2 THEN 'search'
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WHEN 3 THEN 'elasticsearch'
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ELSE 'default'
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END AS "case";
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case
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---------------
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default
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```
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::::{note}
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All result expressions must be of compatible data types. More specifically all result expressions should have a compatible data type with the 1st *non-null* result expression. E.g.:
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for the following query:
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```sql
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CASE WHEN a = 1 THEN null
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WHEN a > 2 THEN 10
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WHEN a > 5 THEN 'foo'
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END
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```
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an error message would be returned, mentioning that **foo** is of data type **keyword**, which does not match the expected data type **integer** (based on result **10**).
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::::
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### Conditional bucketing [sql-functions-conditional-case-groupby-custom-buckets]
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CASE can be used as a GROUP BY key in a query to facilitate custom bucketing and assign descriptive names to those buckets. If, for example, the values for a key are too many or, simply, ranges of those values are more interesting than every single value, CASE can create custom buckets as in the following example:
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```sql
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SELECT count(*) AS count,
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CASE WHEN NVL(languages, 0) = 0 THEN 'zero'
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WHEN languages = 1 THEN 'one'
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WHEN languages = 2 THEN 'bilingual'
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WHEN languages = 3 THEN 'trilingual'
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ELSE 'multilingual'
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END as lang_skills
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FROM employees
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GROUP BY lang_skills
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ORDER BY lang_skills;
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```
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With this query, one can create normal grouping buckets for values *0, 1, 2, 3* with descriptive names, and every value *>= 4* falls into the *multilingual* bucket.
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## `COALESCE` [sql-functions-conditional-coalesce]
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```sql
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COALESCE(
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expression, <1>
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expression, <2>
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...)
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```
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**Input**:
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1. 1st expression
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2. 2nd expression
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…
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**N**th expression
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COALESCE can take an arbitrary number of arguments.
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**Output**: one of the expressions or `null`
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**Description**: Returns the first of its arguments that is not null. If all arguments are null, then it returns `null`.
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```sql
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SELECT COALESCE(null, 'elastic', 'search') AS "coalesce";
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coalesce
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---------------
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elastic
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```
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```sql
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SELECT COALESCE(null, null, null, null) AS "coalesce";
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coalesce
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---------------
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null
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```
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## `GREATEST` [sql-functions-conditional-greatest]
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```sql
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GREATEST(
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expression, <1>
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expression, <2>
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...)
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```
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**Input**:
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1. 1st expression
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2. 2nd expression
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…
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**N**th expression
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GREATEST can take an arbitrary number of arguments and all of them must be of the same data type.
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**Output**: one of the expressions or `null`
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**Description**: Returns the argument that has the largest value which is not null. If all arguments are null, then it returns `null`.
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```sql
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SELECT GREATEST(null, 1, 2) AS "greatest";
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greatest
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---------------
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2
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```
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```sql
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SELECT GREATEST(null, null, null, null) AS "greatest";
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greatest
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---------------
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null
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```
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## `IFNULL` [sql-functions-conditional-ifnull]
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```sql
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IFNULL(
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expression, <1>
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expression) <2>
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```
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**Input**:
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1. 1st expression
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2. 2nd expression
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**Output**: 2nd expression if 1st expression is null, otherwise 1st expression.
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**Description**: Variant of [`COALESCE`](#sql-functions-conditional-coalesce) with only two arguments. Returns the first of its arguments that is not null. If all arguments are null, then it returns `null`.
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```sql
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SELECT IFNULL('elastic', null) AS "ifnull";
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ifnull
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---------------
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elastic
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```
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```sql
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SELECT IFNULL(null, 'search') AS "ifnull";
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ifnull
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---------------
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search
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```
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## `IIF` [sql-functions-conditional-iif]
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```sql
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IIF(
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expression, <1>
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expression, <2>
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[expression]) <3>
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```
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**Input**:
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1. boolean condition to check
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2. return value if the boolean condition evaluates to `true`
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3. return value if the boolean condition evaluates `false`; optional
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**Output**: 2nd expression if 1st expression (condition) evaluates to `true`. If it evaluates to `false` return 3rd expression. If 3rd expression is not provided return `null`.
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**Description**: Conditional function that implements the standard *IF <condition> THEN <result1> ELSE <result2>* logic of programming languages. If the 3rd expression is not provided and the condition evaluates to `false`, `null` is returned.
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```sql
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SELECT IIF(1 < 2, 'TRUE', 'FALSE') AS result1, IIF(1 > 2, 'TRUE', 'FALSE') AS result2;
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result1 | result2
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---------------+---------------
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TRUE |FALSE
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```
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```sql
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SELECT IIF(1 < 2, 'TRUE') AS result1, IIF(1 > 2 , 'TRUE') AS result2;
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result1 | result2
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---------------+---------------
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TRUE |null
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```
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::::{tip}
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**IIF** functions can be combined to implement more complex logic simulating the [`CASE`](#sql-functions-conditional-case) expression. E.g.:
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```sql
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IIF(a = 1, 'one', IIF(a = 2, 'two', IIF(a = 3, 'three', 'many')))
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```
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::::
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## `ISNULL` [sql-functions-conditional-isnull]
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```sql
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ISNULL(
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expression, <1>
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expression) <2>
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```
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**Input**:
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1. 1st expression
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2. 2nd expression
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**Output**: 2nd expression if 1st expression is null, otherwise 1st expression.
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**Description**: Variant of [`COALESCE`](#sql-functions-conditional-coalesce) with only two arguments. Returns the first of its arguments that is not null. If all arguments are null, then it returns `null`.
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```sql
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SELECT ISNULL('elastic', null) AS "isnull";
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isnull
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---------------
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elastic
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```
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```sql
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SELECT ISNULL(null, 'search') AS "isnull";
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isnull
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---------------
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search
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```
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## `LEAST` [sql-functions-conditional-least]
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```sql
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LEAST(
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expression, <1>
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expression, <2>
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...)
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```
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**Input**:
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1. 1st expression
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2. 2nd expression
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…
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**N**th expression
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LEAST can take an arbitrary number of arguments and all of them must be of the same data type.
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**Output**: one of the expressions or `null`
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**Description**: Returns the argument that has the smallest value which is not null. If all arguments are null, then it returns `null`.
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```sql
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SELECT LEAST(null, 2, 1) AS "least";
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least
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---------------
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1
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```
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```sql
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SELECT LEAST(null, null, null, null) AS "least";
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least
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---------------
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null
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```
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## `NULLIF` [sql-functions-conditional-nullif]
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```sql
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NULLIF(
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expression, <1>
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expression) <2>
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```
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**Input**:
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1. 1st expression
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2. 2nd expression
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**Output**: `null` if the 2 expressions are equal, otherwise the 1st expression.
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**Description**: Returns `null` when the two input expressions are equal and if not, it returns the 1st expression.
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```sql
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SELECT NULLIF('elastic', 'search') AS "nullif";
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nullif
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---------------
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elastic
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```
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```sql
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SELECT NULLIF('elastic', 'elastic') AS "nullif";
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nullif:s
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---------------
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null
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```
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## `NVL` [sql-functions-conditional-nvl]
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```sql
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NVL(
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expression, <1>
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expression) <2>
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```
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**Input**:
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1. 1st expression
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2. 2nd expression
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**Output**: 2nd expression if 1st expression is null, otherwise 1st expression.
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**Description**: Variant of [`COALESCE`](#sql-functions-conditional-coalesce) with only two arguments. Returns the first of its arguments that is not null. If all arguments are null, then it returns `null`.
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```sql
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SELECT NVL('elastic', null) AS "nvl";
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nvl
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---------------
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elastic
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```
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```sql
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SELECT NVL(null, 'search') AS "nvl";
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nvl
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---------------
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search
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```
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