MaxCompute SQL allows you to convert data types. Two conversion methods are supported: explicit conversion and implicit conversion.

Explicit conversion

An explicit conversion uses the CAST function to convert the data type of a value. The following table describes the rules of explicit conversions supported by MaxCompute SQL. For more information about the CAST function, see Other functions.
From/To BIGINT DOUBLE STRING DATETIME BOOLEAN DECIMAL FLOAT
BIGINT N/A Y Y N Y Y Y
DOUBLE Y N/A Y N Y Y Y
STRING Y Y N/A Y Y Y Y
DATETIME N N Y N/A N N N
BOOLEAN Y Y Y N N/A Y Y
DECIMAL Y Y Y N Y N/A Y
FLOAT Y Y Y N Y Y N/A

Y indicates that the conversion is supported. N indicates that the conversion is not supported. N/A indicates that the conversion is not required. An error is returned if an unsupported explicit conversion is performed.

Examples
SELECT CAST(user_id AS DOUBLE) AS new_id;
SELECT CAST('2015-10-01 00:00:00' AS DATETIME) AS new_date;
SELECT CAST(ARRAY(1,2,3) AS ARRAY<STRING>);
SELECT CONCAT_WS(',', CAST(ARRAY(1, 2) AS ARRAY<STRING>));
Instructions and limits
  • If a value of the DOUBLE type is converted into that of the BIGINT type, digits after the decimal point are removed. For example, you can execute CAST(1.6 AS BIGINT) = 1 to convert 1.6 of the DOUBLE Type to 1 of the BIGINT type.
  • If a value of the STRING type that meets the format of the DOUBLE type is converted into the BIGINT type, the STRING type is first converted into the DOUBLE type and then to the BIGINT type. During the conversion, the digits after the decimal point are removed. For example, you can execute CAST("1.6" AS BIGINT) = 1 to convert 1.6 of the STRING type to 1 of the BIGINT type.
  • If a value of the STRING type that meets the format of the BIGINT type is converted into the DOUBLE type, one digit is retained after the decimal point. For example, you can execute CAST("1" AS DOUBLE) = 1.0 to convert 1 of the STRING type to 1.0 of the DOUBLE type.
  • The default format yyyy-mm-dd hh:mi:ss is used during the conversion that involves the DATETIME type.
  • Some data types cannot be explicitly converted, but can be converted by using SQL built-in functions. For example, you can use the TO_CHAR function to convert the BOOLEAN type into the STRING type. For more information, see TO_CHAR. You can also use the TO_DATE function to convert the STRING type into the DATETIME type. For more information, see TO_DATE.
  • If the value of the DECIMAL type exceeds its value range, the CAST STRING TO DECIMAL operation may cause errors, such as overflow of the most significant bit or removal of the least significant bit.
  • A conversion from the DECIMAL type to the DOUBLE or FLOAT type results in a loss of precision. In scenarios where high precision is required, for example, when you calculate the bill amount or premium rate, we recommend that you retain the DECIMAL type.
  • MaxCompute allows you to convert complex data types. Implicit conversions between complex data types can be implemented only when their subtypes support implicit conversions. Explicit conversions between complex data types can be implemented only when their subtypes support explicit conversions. If you convert the STRUCT type, field names can be inconsistent, but the number of fields must be consistent and these fields must support implicit or explicit conversions. Examples:
    • ARRAY<BIGINT> can be implicitly or explicitly converted to ARRAY<STRING>.
    • ARRAY<BIGINT> can be explicitly converted to ARRAY<INT>. Implicit conversions are not supported.
    • ARRAY<BIGINT> cannot be implicitly or explicitly converted to ARRAY<DATETIME>.
    • STRUCT<a:BIGINT,b:INT> can be implicitly converted to STRUCT<col1:STRING,col2:BIGINT>. Implicit or explicit conversions to STRUCT<a:STRING> are not supported.

Implicit conversion and its application scope

An implicit conversion allows MaxCompute to automatically convert data types based on the context and predefined rules. The following table describes the rules of implicit conversions supported by MaxCompute.
From/To BOOLEAN TINYINT SMALLINT INT BIGINT FLOAT
BOOLEAN Y N N N N N
TINYINT N Y Y Y Y Y
SMALLINT N N Y Y Y Y
INT N N Y Y Y Y
BIGINT N N N N Y Y
FLOAT N N N N Y Y
From/To DOUBLE DECIMAL STRING VARCHAR TIMESTAMP BINARY
DOUBLE Y Y Y Y N N
DECIMAL N Y Y Y N N
STRING Y Y Y Y N N
VARCHAR Y Y N N N/A N/A
TIMESTAMP N N Y Y Y N
BINARY N N N N N Y

Y indicates that the conversion is supported. N indicates that the conversion is not supported. N/A indicates that the conversion is not required. An error is returned if an unsupported implicit conversion is performed or if a conversion fails.

Note
  • MaxCompute V2.0 introduces the methods to define constants of the DECIMAL and DATETIME types. For example, 100BD indicates value 100 of the DECIMAL type. 2017-11-11 00:00:00 indicates a constant of the DATETIME type. Constants can be directly defined in the VALUES clauses and tables.
  • If an implicit conversion is used, MaxCompute automatically converts data types based on context. If the types do not match, you can use the CAST function to explicitly convert data types.
  • Implicit conversion rules apply to specific scopes. In specific scenarios, only part of the rules take effect.
Example
SELECT user_id+age+'12345', CONCAT(user_name,user_id,age) FROM user;
Implicit conversions with different operators:
  • Implicit conversions with relational operators

    Relational operators include =, <>, <, ≤, >, ≥, IS NULL, IS NOT NULL, LIKE, RLIKE, and IN. The rules that use LIKE, RLIKE, and IN are different from those that use other relational operators. The rules described in this section do not apply to the three operators.

    The following table describes the rules of implicit conversions when data of different types is used for relational operations.
    From/To BIGINT DOUBLE STRING DATETIME BOOLEAN DECIMAL
    BIGINT N/A DOUBLE DOUBLE N N DECIMAL
    DOUBLE DOUBLE N/A DOUBLE N N DECIMAL
    STRING DOUBLE DOUBLE N/A DATETIME N DECIMAL
    DATETIME N N DATETIME N/A N N
    BOOLEAN N N N N N/A N
    DECIMAL DECIMAL DECIMAL DECIMAL N N N/A
    Note
    • If two values you want to compare do not support implicit conversions, the relational operation cannot be completed and an error is returned.
    • For more information about relational operators, see Operators.
  • Implicit conversions with special relational operators
    Special relational operators are LIKE, RLIKE, and IN.
    • Syntax of LIKE and RLIKE
      source LIKE pattern;  
      source RLIKE pattern;
      Note
      • The source and pattern parameters of LIKE and RLIKE must be of the STRING type.
      • Other types can neither be involved in the operation nor be implicitly converted into the STRING type.
    • Syntax of IN
      key IN (value1, value2, ...)
      Note
      • The data types in the value list next to IN must be consistent.
      • If the data types of values include BIGINT, DOUBLE, and STRING, convert the values of the BIGINT and STRING types into the DOUBLE type. If the data types of values include DATETIME and STRING, convert the values of the STRING type into the DATETIME type. Conversions between other data types are not allowed.
  • Implicit conversions with arithmetic operators
    Arithmetic operators include: +, -, *, /, and %. The following rules apply to implicit conversions with these operators.
    • Only the values of the STRING, BIGINT, DOUBLE, and DECIMAL types can be used for arithmetic operations.
    • Values of the STRING type are implicitly converted into the DOUBLE type before the arithmetic operations.
    • If values of the BIGINT and DOUBLE types are used for the arithmetic operations, the value of the BIGINT type is implicitly converted into the DOUBLE type.
    • The values of the DATETIME and BOOLEAN types cannot be used for arithmetic operations.
  • Implicit conversions with logical operators
    Logical operators include AND, OR, and NOT. The following rules apply to implicit conversions with these operators:
    • Only the values of the BOOLEAN type can be used for logical operations.
    • Value of other types cannot be used for logical operations or implicitly converted.

Implicit conversions with built-in functions

MaxCompute SQL provides a variety of built-in functions. These functions can be used to calculate one or more columns of a specific row and provide data of a specific type. The following rules apply to implicit conversions with these functions:
  • If you call a built-in function and the data type of an input parameter is different from that defined in the function, the data type of the input parameter is converted into the function-defined data type.
  • The parameters of each built-in function in MaxCompute SQL have different requirements for implicit conversions. For more information, see Built-in functions.

Implicit conversions with CASE WHEN

For more information about CASE WHEN, see CASE WHEN expressions. The following rules apply to implicit conversions with CASE WHEN:
  • If the return values are of the BIGINT and DOUBLE types, all the values are converted into the DOUBLE type.
  • If the return values include those of the STRING type, all the values are converted into the STRING type. If a conversion, such as a conversion from BOOLEAN to STRING fails, an error is returned.
  • Conversions between other data types are not allowed.

Conversions between the STRING and DATETIME types

MaxCompute supports conversions between the STRING and DATETIME types. The format yyyy-mm-dd hh:mi:ss is used during the conversions.

Time unit String (not case-sensitive) Valid value
Year yyyy 0001-9999
Month mm 01-12
Day dd 01-28|29|30|31
Hour hh 00-23
Minute mi 00-59
Second ss 00-59
Note
  • If the first digit of the value range of each time unit is 0, 0 cannot be omitted. For example, 2014-1-9 12:12:12 is an invalid DATETIME format and it cannot be converted from the STRING type to the DATETIME type. It must be written as 2014-01-09 12:12:12.
  • Only the STRING type that meets the preceding format requirements can be converted into the DATETIME type. For example, CAST("2013-12-31 02:34:34" AS DATETIME) converts 2013-12-31 02:34:34 of the STRING type to the DATETIME type. Similarly, if the DATETIME type is converted into the STRING type, the yyyy-mm-dd hh:mi:ss format is automatically used after the conversion.
Conversions using the following or similar CAST functions fail and errors are returned.
CAST("2013/12/31 02/34/34" AS DATETIME)  
CAST("20131231023434" AS DATETIME)  
CAST("2013-12-31 2:34:34" AS DATETIME)
Some thresholds are based on the number of days in a month. If the number of days in the month is exceeded, an error is returned.
CAST("2013-02-29 12:12:12" AS DATETIME)      -- An error is returned because February 29, 2013 does not exist. 
CAST("2013-11-31 12:12:12" AS DATETIME)      -- An error is returned because November 31, 2013 does not exist. 

MaxCompute provides the TO_DATE function to convert the STRING type that does not meet the format of the DATETIME type to the DATETIME type. For more information, see TO_DATE.