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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
Aggregate Expressions
MAX Function
The MAX function returns the largest value in the specified column or expression.

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
      - 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
        Types of Aggregate Expressions
        Subset of Expressions Valid in an Aggregate Expression
        Including or excluding duplicates in the result set
        You can use the ALL, DISTINCT, or UNIQUE keywords to qualify the scope of an aggregate function.
        AVG Function
        The AVG function returns the average of all values in the specified column or expression.
        Overview of COUNT Functions
        The COUNT function is actually a set of functions that enable you to count column values in different ways, according to arguments after the COUNT keyword.
        COUNT(*) function
        The COUNT (*) function returns the number of rows that satisfy the WHERE clause of a SELECT statement.
        COUNT DISTINCT and COUNT UNIQUE functions
        The COUNT DISTINCT and COUNT UNIQUE functions return unique values.
        COUNT column Function
        Arguments to the COUNT Functions
        MAX Function
        The MAX function returns the largest value in the specified column or expression.
        MIN Function
        SUM Function
        RANGE Function
        The RANGE function returns the range of values for a numeric column expression argument.
        STDEV Function
        VARIANCE Function
        The VARIANCE function returns an estimate of the population variance, as the standard deviation squared.
        Error Checking in ESQL/C
        Summary of Aggregate Function Behavior
        User-Defined Aggregates
        You can create your own aggregate expressions with the CREATE AGGREGATE statement and then invoke these aggregates wherever you can invoke the built-in aggregates.
        Aggregate expressions in grid queries
        Aggregate expressions in grid queries require the database server that issues the grid query to combine aggregate results from multiple grid servers.
    - 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.

MAX Function

The MAX function returns the largest value in the specified column or expression.

Using the DISTINCT keyword does not change the results. The query in the following example finds the most expensive item that is in stock but has not been ordered:

SELECT MAX(unit_price) FROM stock
   WHERE NOT EXISTS (SELECT * FROM items 
      WHERE stock.stock_num = items.stock_num AND
      stock.manu_code = items.manu_code);

NULLs are ignored unless every value in the column is NULL. If every column value is NULL, the MAX function returns a NULL for that column.