Conditional Expression Functions

Conditional Expression Functions

• coalesce(expr1, expr2, ..., exprn)

  Description:

  Returns the first of its arguments that are not null.

  COALESCE(expr1, expr2) is equivalent to CASE WHEN expr1 IS NOT NULL THEN expr1 ELSE expr2 END.

  Example:

  1 2 3 4 5

  openGauss=# SELECT coalesce(NULL,'hello'); coalesce ---------- hello (1 row)

  Note:

  • Null is returned only if all parameters are null.
  • This value is replaced by the default value when data is displayed.
  • Like a CASE expression, COALESCE only evaluates the parameters that are needed to determine the result. That is, parameters to the right of the first not-NULL parameter are not evaluated.
• decode(base_expr, compare1, value1, Compare2,value2, ... default)

  Description: Compares base_expr with each compare(n) and returns value(n) if they are matched. If base_expr does not match each compare(n), the default value is returned.

  Example:

  1 2 3 4 5

  openGauss=# SELECT decode('A','A',1,'B',2,0); case ------ 1 (1 row)

• nullif(expr1, expr2)

  Description: Returns NULL only when expr1 is equal to expr2. Otherwise, expr1 is returned.

  nullif(expr1, expr2) is equivalent to CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END.

  Example:

  1 2 3 4 5

  openGauss=# SELECT nullif('hello','world'); nullif -------- hello (1 row)

  Note:

  Assume the two parameter data types are different:

  • If implicit conversion exists between the two data types, implicitly convert the parameter of lower priority to this data type using the data type of higher priority. If the conversion succeeds, computation is performed. Otherwise, an error is returned. Example:

    1 2 3 4 5

    openGauss=# SELECT nullif('1234'::VARCHAR,123::INT4); nullif -------- 1234 (1 row)

    1 2	openGauss=# SELECT nullif('1234'::VARCHAR,'2012-12-24'::DATE); ERROR: invalid input syntax for type timestamp: "1234"

  • If implicit conversion is not applied between two data types, an error is displayed. Example:

    1 2 3 4 5

    openGauss=# SELECT nullif(TRUE::BOOLEAN,'2012-12-24'::DATE); ERROR: operator does not exist: boolean = timestamp without time zone LINE 1: SELECT nullif(TRUE::BOOLEAN,'2012-12-24'::DATE) FROM sys_dummy; ^ HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts.

• nvl( expr1 , expr2 )

  Description:

  • If the value of expr1 is NULL, the value of expr2 is returned.
  • If the value of expr1 is not NULL, the value of expr1 is returned.

  Example:

  1 2 3 4 5

  openGauss=# SELECT nvl('hello','world'); nvl ------- hello (1 row)

  Parameters expr1 and expr2 can be of any data type. If expr1 and expr2 are of different data types, NVL checks whether expr2 can be implicitly converted to expr1. If it can, the expr1 data type is returned. If epr2 cannot be implicitly converted to expr1 but epr1 can be implicitly converted to expr2, the expr2 data type is returned. If no implicit type conversion exists between the two parameters and the parameters are different data types, an error is reported.

• greatest(expr1 [, ...])

  Description: Selects the largest value from a list of any number of expressions.

  Return type:

  Example:

  1 2 3 4 5

  openGauss=# SELECT greatest(1*2,2-3,4-1); greatest ---------- 3 (1 row)

  1 2 3 4 5

  openGauss=# SELECT greatest('HARRY', 'HARRIOT', 'HAROLD'); greatest ---------- HARRY (1 row)

• least(expr1 [, ...])

  Description: Selects the smallest value from a list of any number of expressions.

  Example:

  1 2 3 4 5

  openGauss=# SELECT least(1*2,2-3,4-1); least ------- -1 (1 row)

  1 2 3 4 5

  openGauss=# SELECT least('HARRY','HARRIOT','HAROLD'); least -------- HAROLD (1 row)

• EMPTY_BLOB()

  Description: Initiates a BLOB variable in an INSERT or an UPDATE statement to a NULL value.

  Return type: BLOB

  Example:

  1 2 3 4 5 6

  -- Create a table. openGauss=# CREATE TABLE blob_tb(b blob,id int) DISTRIBUTE BY REPLICATION; -- Insert data. openGauss=# INSERT INTO blob_tb VALUES (empty_blob(),1); -- Drop the table. openGauss=# DROP TABLE blob_tb;

  Note: The length is 0 obtained using DBE_LOB.GET_LENGTH.