User-defined variables

This topic describes how to declare and use user-defined variables.

These variables are used to store specific values that are referenced in later SQL statements, thereby simplifying the writing of SQL statements and avoiding duplicate computation.

Usage notes

• User-defined variables are variables, which can be created by the user and exist in the session. This means that no one can access user-defined variables that have been declared by another user, and when the session is closed these variables expire.
• CelerData does not support using the SHOW statement to display existing user-defined variables.
• The following types of values cannot be declared as user-defined variables: BITMAP, HLL, PERCENTILE, and ARRAY. User-defined variables of the JSON type are converted to the STRING type for storage.

Declare user-defined variables

Syntax

SET @var_name = expr [, ...];

NOTE

• All variables must be preceded by a single at sign (@).
• Multiple variables can be declared in the same SET statement and need to be separated with commas (,).
• You can declare the same variable multiple times. The newly declared value overwrites the original value.
• If an undeclared variable is used, the value of the variable is NULL by default, and the NULL type is STRING.

Parameters

Parameter	Required	Description
var_name	Yes	The name of the user-defined variable. The naming conventions are as follows: The name can contain letters, digits (0-9), and underscores (_). The name cannot exceed 64 characters in length. You can declare a string as a user-defined variable, such as @'my-var', @"my-var", and @`my-var`. User-defined variables of the STRING type can contain characters other than letters, digits, and underscores (_), such as periods (.).
expr	Yes	The value of the user-defined variable. You can specify a number (such as 43) or a complex expression (such as the value returned by a SELECT statement) for this parameter. The data type of the variable is the same as the data type of the result returned by the expression.

Examples

Example 1: Declare a number as a user-defined variable.

SET @var = 43;

Example 2: Declare the value returned by a SELECT query as a user-defined variable.

SET @var = (SELECT SUM(v1) FROM test);

Example 3: Declare multiple user-defined variables in the same SET statement.

SET @v1=1, @v2=2;

Use user-defined variables in SQL

• Simplifies the writing of SQL statements. For example, when you execute the following SELECT statement, the system parses @var as 1.

  SET @var = 1;
  SELECT @var, v1 from test;

• Avoids duplicate computation. For example, when you execute the following SELECT statement, the system parses @var as the result returned by the select sum(c1) from tbl command.

  SET @var = (select sum(c1) from tbl);
  SELECT @var, v1 from test;