Conditional Expression Functions

coalesce(expr1, expr2, ..., exprn)

Description: Returns the first non-NULL value in the parameter list.

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

Example:

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

Note:

• NULL is returned only if all parameters are NULL.
• It is often used to replace NULL with the default value.
• 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 non-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 no matching result is found, the default value is returned.

Example:

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

Note: Operations on XML data are not supported.

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.

The nullif(expr1, expr2) function is a mapping function. Therefore, the corresponding function definition cannot be found in the pg_proc system catalog.

Example:

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

Note: Operations on XML data are not supported.

Assume the two parameter data types are different:

• If applicable, the lower-priority data type will be implicitly converted into the higher-priority one. Upon successful conversion, the calculation will begin. If unsuccessful, an error will be reported. Below is an example:

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

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

• If implicit conversion is not applicable between two data types, an error will be reported. Below is an example:

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  gaussdb=# SELECT nullif(1::bit, '1'::MONEY); ERROR: operator does not exist: bit = money LINE 1: SELECT nullif(1::bit, '1'::MONEY); ^ HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts. CONTEXT: referenced column: nullif

nvl(expr1, expr2)

Description:

• Returns the value of expr2 if expr1 is null.
• Returns the value of expr1 if it is not null.

Example:

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

Note: expr1 and expr2 can be of any data type. If they have different data types, the function will check whether expr2 can be implicitly converted into expr1. If expr2 can be implicitly converted into expr1, the function will return the data type of expr1. If expr2 cannot be implicitly converted into expr1 but expr1 can be implicitly converted into expr2, the function will return the data type of expr2. If there is no possibility of implicit conversion in either direction, the function will report an error.

nvl2(expr1, expr2, expr3)

Description:

• Returns the value of expr3 if expr1 is null.
• Returns the value of expr2 if expr1 is not null.
  

  This function takes effect when a_format_version is 10c and a_format_dev_version is s1.

Example:

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gaussdb=# SELECT nvl2('hello','world','other');
  case  
-------
 world
(1 row)

Note: The expr2 and expr3 parameters can be of any type. If the last two parameters of NVL2 have different types, the function will check whether expr3 can be implicitly converted into expr2. If not, an error will be reported. If the first parameter is of the numeric type, the function converts this parameter and other parameters to the numeric type, and then compares them. If the parameters cannot be converted, an error message is displayed. If the first parameter is of another type, the function converts other parameters to the type of the first parameter for comparison. If the parameters cannot be converted, an error message is displayed.

greatest(expr1 [, ...])

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

Return type:

Example:

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

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

Note: Operations on XML data are not supported.

When a_format_version is 10c and a_format_dev_version is s1, this function operates as follows:

1. If the value of any parameter is null, the function returns null.
2. If the first parameter is of the numeric type, the function converts this parameter and other parameters to the numeric type, and then compares them. If the parameters cannot be converted, an error message is displayed. If the first parameter is of another type, the function converts other parameters to the type of the first parameter for comparison. If the parameters cannot be converted, an error message is displayed.

least(expr1 [, ...])

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

Example:

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

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

Note: Operations on XML data are not supported.

When a_format_version is 10c and a_format_dev_version is s1, this function operates as follows:

1. If the value of any parameter is null, the function returns null.
2. If the first parameter is of the numeric type, the function converts this parameter and other parameters to the numeric type, and then compares them. If the parameters cannot be converted, an error message is displayed. If the first parameter is of another type, the function converts other parameters to the type of the first parameter for comparison. If the parameters cannot be converted, an error message is displayed.

EMPTY_BLOB()

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

Return type: BLOB

Example:

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

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

EMPTY_CLOB()

Description: Initiates a CLOB variable in an INSERT or UPDATE statement to a null value.

This function takes effect when a_format_version is 10c and a_format_dev_version is s1.

Return type: CLOB

Example:

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-- Create a table.
gaussdb=# CREATE TABLE clob_tb(c clob,id int);
-- Insert data.
gaussdb=# INSERT INTO clob_tb VALUES (empty_clob(),1);
-- Delete the table.
gaussdb=# DROP TABLE clob_tb;

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

lnnvl(condition)

Description: Checks the condition in the WHERE clause of a query statement. If the condition is true, false is returned. If the condition is unknown or false, true is returned.

condition: The value must be a logical expression. However, it cannot be used in composite conditions with keywords such as AND, OR, and BETWEEN.

Return type: Boolean

Example:

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-- Create a table.
gaussdb=# CREATE TABLE student_demo (name VARCHAR2(20), grade  NUMBER(10,2));
CREATE TABLE

-- Insert data.
gaussdb=# INSERT INTO student_demo VALUES ('name0',0);
INSERT 0 1
gaussdb=# INSERT INTO student_demo VALUES ('name1',1);
INSERT 0 1
gaussdb=# INSERT INTO student_demo VALUES ('name2',2);
INSERT 0 1

-- Call lnnvl.
gaussdb=# SELECT * FROM student_demo WHERE LNNVL(name = 'name1');
 name  | grade 
-------+-------
 name0 |  0.00
 name2 |  2.00
(2 rows)

-- Delete the table.
gaussdb=# drop table student_demo;
DROP TABLE

The lnnvl function is supported only when a_format_version is 10c and a_format_dev_version is s2.

isnull(expr)

Description: Checks whether expr is NULL. If so, the function will return true. If not, the function will return false. The isnull function is compatible in full mode.

Parameter: Any type of input is supported.

Return type: Boolean

Example:

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gaussdb=# SELECT isnull(null);
 isnull 
--------
 t
(1 row)

gaussdb=# SELECT isnull(1);
 isnull 
--------
 f
(1 row)

if(expr1, expr2, expr3)

Description: Checks the value of expr1. If expr1 is true, expr2 is returned. Otherwise, expr3 is returned.

if(expr1, expr2, expr3) is logically equivalent to CASE WHEN expr1 THEN expr2 ELSE expr3 END.

Parameters: See Table 1.

Return value type: related to the input parameter type. For details about the output rules, see the description.

• if(expr1, expr2, expr3) is supported only in MySQL-compatible mode.
• If both b_format_version and b_format_dev_version are '', the return type of the if(expr1, expr2, expr3) function is consistent with that of CASE WHEN expr1 THEN expr2 ELSE expr3 END. If b_format_version is '5.7' and b_format_dev_version is 's1', the return type is determined in the following descending order:
  • If both input parameters are of the same type, the return type will match the input parameter type.
  • If either input parameter is of the SET type, the return type will be TEXT.
  • If either input parameter is of the BLOB type, the return type will be BLOB.
  • If either input parameter is of the TEXT type, the return type will be TEXT.
  • If either input parameter is of the STRING type, the return type will be TEXT.
  • If both input parameters are of the time type, the return type will be TIMESTAMPTZ. If one input parameter is of the time type and the other is not, the return type will be TEXT.
  • If both input parameters are signed integers and one input parameter is of the BIGINT type, the return value is of the BIGINT type; otherwise, the return value is of the INT type.
  • If both input parameters are unsigned integers and one input parameter is of the UNSIGNED BIGINT type, the return value is of the UNSIGNED BIGINT type; otherwise, the return value is of the UNSIGNED INT type.
  • If the two input parameters are of the floating point type and integer type respectively, the return value is of the floating point type.
  • Except the preceding NUMERIC type combinations, the return value for other NUMERIC type combinations is of the NUMERIC type.
  • If implicit conversion is not applicable between the input parameter type and the return type, an error will be reported.
• If the input parameter expr1 is null, the returned result is the same as when expr1 is FALSE.
• The if(expr1, expr2, expr3) function is a mapping function. Therefore, the corresponding function definition cannot be found in the pg_proc system catalog.

Example:

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-- expr1 is an expression.
gaussdb=# select if(2>3, 'true', 'false');
 case
-------
 false
(1 row)

-- If the input parameter expr1 is NULL, the result is the same as that when expr1 is false, and the value of expr2 is returned.
gaussdb=# select if(null, 'not null', 'is null');
  case
---------
 is null
(1 row)

ifnull(expr1, expr2)

Description: Returns the value of expr2 if expr1 is NULL; otherwise, returns the value of expr1.

ifnull(expr1, expr2) is logically equivalent to nvl(expr1, expr2).

Parameter: any type.

Return value type: related to the input parameter type. For details about the output rules, see the description.

• ifnull(expr1, expr2) is supported only in MySQL-compatible mode.
• If both b_format_version and b_format_dev_version are '', the return type of the ifnull(expr1, expr2) function is consistent with that of nvl(expr1, expr2). If b_format_version is '5.7' and b_format_dev_version is 's1', the return type is determined in the following descending order:
  • If both input parameters are of the same type, the return type will match the input parameter type.
  • If either input parameter is of the SET type, the return type will be TEXT.
  • If either input parameter is of the BLOB type, the return type will be BLOB.
  • If either input parameter is of the TEXT type, the return type will be TEXT.
  • If either input parameter is of the STRING type, the return type will be TEXT.
  • If both input parameters are of the time type, the return type will be TIMESTAMPTZ. If one input parameter is of the time type and the other is not, the return type will be TEXT.
  • If one input parameter is of the FLOAT4 type and the other NUMERIC, the return type will be DOUBLE.
  • If both input parameters are NUMERIC, the higher-precision type will take precedence. For example, if one input parameter is TINYINT and the other is INT, the return type will be INT.
  • If implicit conversion is not applicable between the input parameter type and the return type, an error will be reported.
• If both input parameters are null, the function will return null. Otherwise, it will return the first non-null value.
• The ifnull(expr1, expr2) function is a mapping function. Therefore, the corresponding function definition cannot be found in the pg_proc system catalog.

Example:

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-- If the input parameter is an empty string, the output is an empty string instead of NULL.
gaussdb=# SELECT ifnull('', null) is null as a;
 a
---
 f
(1 row)

-- If both input parameters are NULL, the output is NULL.
gaussdb=# SELECT ifnull(null, null) is null as a;
 a
---
 t
(1 row)

-- If the input parameters are NULL and a character string, the output is the first non-null value.
gaussdb=# SELECT ifnull(null, 'A') as a;
 a
---
 A
(1 row)