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SQL programming
You can use the HCL Informix® implementation of the SQL language to develop applications for Informix database servers.
Guide to SQL: Syntax
The Informix® Guide to SQL: Syntax describes the syntax of the statements, data types, expressions, operators, and built-in functions of the Informix dialect of the SQL language.
Data types and expressions
These topics describe the data types and expressions that Informix®supports.
Expression
Conditional Expressions
CASE Expressions
The CASE expression allows an SQL statement such as the SELECT statement to return one of several possible results, depending on which of several conditions evaluates to true.

SQL programming
You can use the HCL Informix® implementation of the SQL language to develop applications for Informix database servers.
- Guide to SQL: Syntax
  The Informix® Guide to SQL: Syntax describes the syntax of the statements, data types, expressions, operators, and built-in functions of the Informix dialect of the SQL language.
  - Overview of SQL syntax
    These topics provide an overview about how to use the SQL statements, SPL statements, and syntax segments.
  - SQL statements
    This chapter describes the syntax and semantics of SQL statements that are recognized by Informix®.
  - SPL statements
    These topics describe Stored Procedure Language (SPL) statements, which you use to write SPL routines. You can store these routines in the database as user-defined routines (UDRs).
  - Data types and expressions
    These topics describe the data types and expressions that Informix®supports.
    - Scope of Segment Descriptions
    - Use of Segment Descriptions
    - Data type and expression segments
      Data type and expression segments can appear in SQL statements.
    - Collection Subquery
    - Condition
    - Data Type
      The Data Type segment specifies the data type of a column, of a component of a collection, of a field within a ROW type, of a routine parameter, or of a value returned by an expression or by a cast function. Use this segment whenever you see a reference to a data type in a syntax diagram.
    - DATETIME Field Qualifier
      Use a DATETIME Field Qualifier to specify the largest and smallest unit of time in a DATETIME column or value. Use this segment whenever you see a reference to a DATETIME Field Qualifier in a syntax diagram.
    - Expression
      - List of Expressions
        Each category of SQL expression includes many individual expressions.
      - Arithmetic Operators
      - Bitwise Logical Functions
      - Concatenation Operator
      - CAST Expressions
        Use the CAST (... AS ...) keywords or the double-colon ( :: ) cast operator to convert the data type of the value of an expression to some other target data type.
      - Column Expressions
        A column expression specifies a data value in a column in the database, or a substring of the value, or a field within a ROW-type column, or a field in a BSON column.
      - Conditional Expressions
        CASE Expressions
        The CASE expression allows an SQL statement such as the SELECT statement to return one of several possible results, depending on which of several conditions evaluates to true.
        CASE expressions data type compatibility
        In a CASE expression, all the results should be of the same data type or be compatible data types.
        Generic CASE Expressions
        A generic CASE expression tests for a true condition in a WHEN clause. If it finds a true condition, it returns the result specified in the THEN clause.
        Linear CASE Expressions
        COALESCE Function
        The COALESCE function returns the first non-NULL value from a series of expressions.
        NVL Function
        NULLIF Function
        DECODE Function
        The DECODE expression is similar to the CASE expression in that it can print different results depending on the values found in a specified column.
      - Constant Expressions
        Certain expressions that return a fixed value are called constant expressions. These include variant function operators that read the system clock, but that are valid in contexts where literal constants are also valid.
      - Constructor Expressions
      - NULL Keyword
      - Function Expressions
      - Statement-Local Variable Expressions
      - Aggregate Expressions
    - OLAP window expressions
      You can include On-Line Analytical Processing (OLAP) expressions in a SELECT statement to operate on subsets of the rows in the result set of a query or subquery. You can use OLAP window expressions to examine subsets of the qualifying rows for patterns, trends, or exceptions in the data.
    - INTERVAL Field Qualifier
    - Literal Collection
    - Literal DATETIME
      The Literal DATETIME segment specifies a DATETIME value
    - Literal INTERVAL
      The Literal INTERVAL segment specifies a literal INTERVAL value. Use this whenever you see a reference to a literal INTERVAL in a syntax diagram.
    - Literal Number
    - Literal Row
      The Literal Row segment specifies the syntax for literal values of named and unnamed ROW data types.
    - Quoted String
    - Relational Operator
  - Other syntax segments
    These topics describe syntax segments, which are language elements, such as database object names or optimizer directives, that appear as a subdiagram reference in the syntax diagrams of some SQL or SPL statements.
  - Built-In Routines
  - Appendixes
- Guide to SQL: Reference
  The Informix® Guide to SQL: Reference contains the reference information for the system catalog tables, data types, and environment variables of the HCL Informix dialect of the SQL language, as implemented in HCL Informix. These topics also include information about the stores_demo, sales_demo, and superstore_demo databases that are included with HCL Informix.
- Guide to SQL: Tutorial
  The Informix® Guide to SQL: Tutorial shows how to use basic and advanced structured query language (SQL) to access and manipulate the data in your databases. It discusses the data manipulation language (DML) statements as well as triggers and stored procedure language (SPL) routines, which DML statements often use.

CASE Expressions

The CASE expression allows an SQL statement such as the SELECT statement to return one of several possible results, depending on which of several conditions evaluates to true.

The CASE expression has two forms: generic CASE expressions and linear CASE expressions.

^{(explicit id case002a)
case002a} ^{(explicit id case003)
case003}

CASE Expressions

{ <Generic CASE Expression> ^[] | <Linear CASE Expression>^[] }

You must include at least one WHEN clause in the CASE expression. Subsequent WHEN clauses and the ELSE clause are optional. You can use a generic or linear CASE expression wherever you can use a column expression in an SQL statement (for example, in the Projection clause a SELECT statement).

Expressions in the search condition or the result value expression can contain subqueries, and you can nest a CASE expression in another CASE expression.

When a CASE expression appears in an aggregate expression, you cannot use aggregate functions in the CASE expression.

You can specify a trigger-type Boolean operator (DELETING, INSERTING, SELECTING, or UPDATING) as a condition in a CASE expression only within a trigger routine.

The following query fragment declares aliases for two aggregate column expressions:

SELECT . . . 
   SUM(orders.ship_weight) as o2,
   COUNT(DISTINCT 
         CASE WHEN orders.backlog MATCHES 'n' 
              THEN orders.order_num END ) AS o3, 
   . . .

Here the argument to SUM is a DECIMAL(8,2) column value, and the COUNT DISTINCT aggregate takes a CASE expression as its argument.

Do not confuse CASE expressions with the CASE statement of SPL, which supports different syntax and functionality.

¹ See Generic CASE Expressions

² See Linear CASE Expressions