# Data types

In Kuiper, each column or an expression has a related data type. A data type describes (and constrains) the set of values that a column of that type can hold or an expression of that type can produce.



## Supported data types

Below is the list of data types supported.

| #    | Data type | Description                                                  |
| ---- | --------- | ------------------------------------------------------------ |
| 1    | bigint    | The int type.                                                |
| 2    | float     | The float type.                                              |
| 3    | string    | Text values, comprised of Unicode characters.                |
| 4    | datetime  | datatime type - *Currently it's NOT supported yet*.          |
| 5    | boolean   | The boolean type, the value could be ``true`` or ``false``.  |
| 6    | array     | The array type, can be any types from simple data or struct type (#1 - #5, and #7). |
| 7    | struct    | The complex type. Set of name/value pairs. Values must be of supported data type. |

## Compatibility of comparison and calculation

There may be binary operations in each sql clause. In this example, `Select temperature * 2 from demo where temperature > 20`, a calculation operation is used in select clause and a comparison operation is used in the where clause. In the binary operations, if incompatible data types are used, a runtime error will happen and send to the sinks.

Array and struct are not supported in any binary operations. The compatibility of other data types are listed in below table. Whereas, the row header is the left operand data type and the column header is the right operand data. The value is the compatibility in which Y stands for yes and N stands for no.

| bigint   | float | string | datetime                  | boolean |
| -------- | ----- | ------ | ------------------------- | ------- |
| bigint   | Y     | Y      | N                         | N       |
| float    | Y     | Y      | N                         | N       |
| string   | N     | N      | Y                         | N       |
| datetime | Y     | Y      | Y, if in the valid format | Y       |
| boolean  | N     | N      | N                         | N       |

 The default format for datetime string is ``"2006-01-02T15:04:05.000Z07:00"``

 For `nil` value, we follow the rules:

  1. Compare with nil always return false
  2. Calculate with nil always return nil

## Type conversions

These are the rules governing *data type conversions*:

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