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[docs] add document conditional-functions (#8339)

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Add conditional-functions.
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steadyBoy
2022-03-09 13:05:13 +08:00
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72
docs/en/sql-reference/sql-functions/conditional-functions/case.md Normal file
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---
{
"title": "case",
"language": "en"
}
---
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
-->
# case
## description
### Syntax
```
CASE expression
WHEN condition1 THEN result1
[WHEN condition2 THEN result2]
...
[WHEN conditionN THEN resultN]
[ELSE result]
END
```
OR
```
CASE WHEN condition1 THEN result1
[WHEN condition2 THEN result2]
...
[WHEN conditionN THEN resultN]
[ELSE result]
END
```
Compare the expression with multiple possible values, and return the corresponding results when matching
## example
```
mysql> select user_id, case user_id when 1 then 'user_id = 1' when 2 then 'user_id = 2' else 'user_id not exist' end test_case from test;
+---------+-------------+
| user_id | test_case |
+---------+-------------+
| 1 | user_id = 1 |
| 2 | user_id = 2 |
+---------+-------------+
mysql> select user_id, case when user_id = 1 then 'user_id = 1' when user_id = 2 then 'user_id = 2' else 'user_id not exist' end test_case from test;
+---------+-------------+
| user_id | test_case |
+---------+-------------+
| 1 | user_id = 1 |
| 2 | user_id = 2 |
+---------+-------------+
```
## keyword
CASE

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docs/en/sql-reference/sql-functions/conditional-functions/coalesce.md Normal file
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---
{
"title": "coalesce",
"language": "en"
}
---
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
-->
# coalesce
## description
### Syntax
`coalesce(expr1, expr2, ...., expr_n)`
Returns the first non empty expression in the parameter (from left to right)
## example
```
mysql> select coalesce(NULL, '1111', '0000');
+--------------------------------+
| coalesce(NULL, '1111', '0000') |
+--------------------------------+
| 1111 |
+--------------------------------+
```
## keyword
COALESCE

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docs/en/sql-reference/sql-functions/conditional-functions/if.md Normal file
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---
{
"title": "if",
"language": "en"
}
---
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
-->
# if
## description
### Syntax
`if(boolean condition, type valueTrue, type valueFalseOrNull)`
Returns valueTrue when condition is true, returns valueFalseOrNull otherwise.
The return type is the type of the result of the valueTrue/valueFalseOrNull expression
## example
```
mysql> select user_id, if(user_id = 1, "true", "false") test_if from test;
+---------+---------+
| user_id | test_if |
+---------+---------+
| 1 | true |
| 2 | false |
+---------+---------+
```
## keyword
IF

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docs/en/sql-reference/sql-functions/conditional-functions/ifnull.md Normal file
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---
{
"title": "ifnull",
"language": "en"
}
---
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
-->
# ifnull
## description
### Syntax
`ifnull(expr1, expr2)`
If the value of expr1 is not null, expr1 is returned, otherwise expr2 is returned
## example
```
mysql> select ifnull(1,0);
+--------------+
| ifnull(1, 0) |
+--------------+
| 1 |
+--------------+
mysql> select ifnull(null,10);
+------------------+
| ifnull(NULL, 10) |
+------------------+
| 10 |
+------------------+
```
## keyword
IFNULL

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docs/en/sql-reference/sql-functions/conditional-functions/nullif.md Normal file
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---
{
"title": "nullif",
"language": "en"
}
---
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
-->
# nullif
## description
### Syntax
`nullif(expr1, expr2)`
If the two parameters are equal, null is returned. Otherwise, the value of the first parameter is returned. It has the same effect as the following `case when`
```
CASE
WHEN expr1 = expr2 THEN NULL
ELSE expr1
END
```
## example
```
mysql> select nullif(1,1);
+--------------+
| nullif(1, 1) |
+--------------+
| NULL |
+--------------+
mysql> select nullif(1,0);
+--------------+
| nullif(1, 0) |
+--------------+
| 1 |
+--------------+
```
## keyword
NULLIF