sqt::PredicateType Concept Reference

#include <sqt/orm/expression/predicate_type.h>

Description

Constrains a type to be a predicate type.

Requirements

• The type must satisfy the sqt::ExpressionLike concept.
• The type must have a static constant Operator of type sqt::PredicateOperator, representing the operator used in the predicate.

Predicates are binary expressions that return a boolean, including comparison expressions and logical expressions. They are used in WHERE decorators of queriers to apply conditions to the SQL statement.

A predicate consists of an operator (specified by the sqt::PredicateOperator enum) and two operands (which must satisfy the sqt::PredicateTermType concept). Operands can also be a predicate, allowing to build a tree structure of predicates.

To create predicates, use the comparison operators (==, !=, <, <=, >, >=) on identifiers (including columns, primary keys and indexes), or logical operators (&&, ||) on other predicates. Consider the following entity type and its table definition:

struct MyEntity {
    int id{};
    std::string name;
};
SQT_TABLE_BEGIN(MyEntity, MyEntity)
SQT_COLUMN_FIELD(id, id)
SQT_COLUMN_FIELD(name, name)
SQT_PRIMARY_KEY(id)
SQT_INDEX(id, name)
SQT_TABLE_END

Examples of creating selecters with predicates:

// Selects entities where the name is not "Unknown".
auto selecter = sqt::DataContext<MyEntity>::MakeSelecter().Where(
    sqt::Table<MyEntity>.name != "Unknown"
);
 
// Selects the entity with the primary key.
auto selecter = sqt::DataContext<MyEntity>::MakeSelecter().Where(
    sqt::Table<MyEntity>.PrimaryKey == 100
);
 
// Selects the entity with the index.
auto selecter = sqt::DataContext<MyEntity>::MakeSelecter().Where(
    sqt::Table<MyEntity>.Index_id_name == std::make_tuple(100, "Unknown")
);
 
// Selects entities with multiple conditions.
auto selecter = sqt::DataContext<MyEntity>::MakeSelecter().Where(
    sqt::Table<MyEntity>.id < 100 && sqt::Table<MyEntity>.name != "Unknown"
);

See also

sqt::ExpressionLike

sqt::PredicateOperator

sqt::PredicateTermType