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# Amazon QLDB 中的 CAST 函數
<a name="ql-functions.cast"></a>

**重要**  
支援終止通知：現有客戶將可以使用 Amazon QLDB，直到 07/31/2025 的支援結束為止。如需詳細資訊，請參閱[將 Amazon QLDB Ledger 遷移至 Amazon Aurora PostgreSQL](https://aws.amazon.com/blogs/database/migrate-an-amazon-qldb-ledger-to-amazon-aurora-postgresql/)。

在 Amazon QLDB 中，使用 `CAST`函數來評估給定表達式的值，並將該值轉換為指定的目標資料類型。如果無法進行轉換，函數會傳回錯誤。

## 語法
<a name="ql-functions.cast.syntax"></a>

```
CAST ( expression AS type )
```

## 引數
<a name="ql-functions.cast.arguments"></a>

*運算式*  
評估為函數轉換的值的欄位名稱或表達式。轉換 Null 值會傳回 Null。此參數可以是任何支援的 [資料類型](ql-reference.data-types.md)。

*type*  
用於轉換的目標資料類型名稱。此參數可以是支援的 之一[資料類型](ql-reference.data-types.md)。

## 傳回類型
<a name="ql-functions.cast.return-type"></a>

類型**引數指定的資料類型。

## 範例
<a name="ql-functions.cast.examples"></a>

下列範例顯示不明類型的傳播 (`NULL` 或 `MISSING`)。

```
CAST(null    AS null)    -- null 
CAST(missing AS null)    -- null 
CAST(missing AS missing) -- missing
CAST(null    AS missing) -- missing
CAST(null    AS boolean) -- null (null AS any data type name results in null)
CAST(missing AS boolean) -- missing (missing AS any data type name results in missing)
```

任何不是未知類型的值都無法轉換為 `NULL`或 `MISSING`。

```
CAST(true AS null)    -- error 
CAST(true AS missing) -- error
CAST(1    AS null)    -- error
CAST(1    AS missing) -- error
```

下列範例顯示投射 `AS boolean`。

```
CAST(true      AS boolean) -- true no-op
CAST(0         AS boolean) -- false
CAST(1         AS boolean) -- true
CAST(`1e0`     AS boolean) -- true (float)
CAST(`1d0`     AS boolean) -- true (decimal)
CAST('a'       AS boolean) -- false
CAST('true'    AS boolean) -- true (SqlName string 'true')
CAST(`'true'`  AS boolean) -- true (Ion symbol `'true'`)
CAST(`'false'` AS boolean) -- false (Ion symbol `'false'`)
```

下列範例顯示投射 `AS integer`。

```
CAST(true   AS integer) -- 1
CAST(false  AS integer) -- 0
CAST(1      AS integer) -- 1
CAST(`1d0`  AS integer) -- 1
CAST(`1d3`  AS integer) -- 1000
CAST(1.00   AS integer) -- 1
CAST(1.45   AS integer) -- 1
CAST(1.75   AS integer) -- 1
CAST('12'   AS integer) -- 12
CAST('aa'   AS integer) -- error
CAST(`'22'` AS integer) -- 22
CAST(`'x'`  AS integer) -- error
```

下列範例顯示投射 `AS float`。

```
CAST(true   AS float) -- 1e0
CAST(false  AS float) -- 0e0
CAST(1      AS float) -- 1e0
CAST(`1d0`  AS float) -- 1e0
CAST(`1d3`  AS float) -- 1000e0
CAST(1.00   AS float) -- 1e0
CAST('12'   AS float) -- 12e0
CAST('aa'   AS float) -- error
CAST(`'22'` AS float) -- 22e0
CAST(`'x'`  AS float) -- error
```

下列範例顯示投射 `AS decimal`。

```
CAST(true   AS decimal) -- 1.
CAST(false  AS decimal) -- 0.
CAST(1      AS decimal) -- 1.
CAST(`1d0`  AS decimal) -- 1. (REPL printer serialized to 1.)
CAST(`1d3`  AS decimal) -- 1d3
CAST(1.00   AS decimal) -- 1.00
CAST('12'   AS decimal) -- 12.
CAST('aa'   AS decimal) -- error
CAST(`'22'` AS decimal) -- 22.
CAST(`'x'`  AS decimal) -- error
```

下列範例顯示投射 `AS timestamp`。

```
CAST(`2001T`                      AS timestamp) -- 2001T
CAST('2001-01-01T'                AS timestamp) -- 2001-01-01T
CAST(`'2010-01-01T00:00:00.000Z'` AS timestamp) -- 2010-01-01T00:00:00.000Z
CAST(true                         AS timestamp) -- error
CAST(2001                         AS timestamp) -- error
```

下列範例顯示投射 `AS symbol`。

```
CAST(`'xx'`                     AS symbol) -- xx (`'xx'` is an Ion symbol)
CAST('xx'                       AS symbol) -- xx ('xx' is a string)
CAST(42                         AS symbol) -- '42'
CAST(`1e0`                      AS symbol) -- '1.0'
CAST(`1d0`                      AS symbol) -- '1'
CAST(true                       AS symbol) -- 'true'
CAST(false                      AS symbol) -- 'false'
CAST(`2001T`                    AS symbol) -- '2001T'
CAST(`2001-01-01T00:00:00.000Z` AS symbol) -- '2001-01-01T00:00:00.000Z'
```

下列範例顯示投射 `AS string`。

```
CAST(`'xx'`                     AS string) -- "xx" (`'xx'` is an Ion symbol)
CAST('xx'                       AS string) -- "xx" ('xx' is a string)
CAST(42                         AS string) -- "42"
CAST(`1e0`                      AS string) -- "1.0"
CAST(`1d0`                      AS string) -- "1"
CAST(true                       AS string) -- "true"
CAST(false                      AS string) -- "false"
CAST(`2001T`                    AS string) -- "2001T"
CAST(`2001-01-01T00:00:00.000Z` AS string) -- "2001-01-01T00:00:00.000Z"
```

下列範例顯示投射 `AS struct`。

```
CAST(`{ a: 1 }` AS struct) -- {a:1}
CAST(true       AS struct) -- err
```

下列範例顯示投射 `AS list`。

```
CAST(`[1, 2, 3]`        AS list) -- [1,2,3]
CAST(<<'a', { 'b':2 }>> AS list) -- ["a",{'b':2}]
CAST({ 'b':2 }          AS list) -- error
```

下列範例是可執行的陳述式，其中包含前面的一些範例。

```
SELECT CAST(true AS integer) FROM << 0 >>            -- 1
SELECT CAST('2001-01-01T' AS timestamp) FROM << 0 >> -- 2001-01-01T
SELECT CAST('xx' AS symbol) FROM << 0 >>             -- xx
SELECT CAST(42 AS string) FROM << 0 >>               -- "42"
```

## 相關函數
<a name="ql-functions.cast.related"></a>
+ [TO\$1STRING](ql-functions.to_string.md)
+ [TO\$1TIMESTAMP](ql-functions.to_timestamp.md)