Triggers: Triggers are powerful and can easily and reliably implement many complex functions, so why should they be used with caution? Triggers themselves are not inherently flawed, but their misuse can cause difficulties in database and application maintenance. In database operations, we can implement data manipulation through relations, triggers, stored procedures, applications, etc. At the same time, rules, constraints, and default valuesare also important guarantees of data integrity. If we rely too heavily on triggers, it will inevitably affect the database structure and increase the complexity of maintenance.
A trigger is a method provided by SQL Server for programmers and data analysts to ensure data integrity. It is a special stored procedure related to table events. Its execution is not called by the program or started manually, but is triggered by events, such as when an operation (insert, delete, update) is performed on a table. Triggers are often used to strengthen data integrity constraints and business rules. Triggers can be found in the DBA_TRIGGERS and USER_TRIGGERS data dictionaries. In SQL3, a trigger is a statement that can be automatically executed by the system to modify the database. Triggers can query other tables and can contain complex SQL statements. They are primarily used to enforce complex business rules or requirements. For example, you can control whether a new order is allowed based on a customer's current account status.
Triggers can also be used to enforce referential integrity, preserving the relationships defined between multiple tables when rows are added, updated, or deleted. However, the best way to enforce referential integrity is to define primary and foreign key constraints in the related tables. If a database graph is used, relationships can be created between tables to automatically create foreign key constraints. The only difference between triggers and stored procedures is that triggers cannot execute EXECUTE statement calls; instead, they are automatically triggered when a user executes a Transact-SQL statement.
DML Triggers:When data in a database table changes, including insert, update, and delete operations, a corresponding DML trigger is automatically executed if a DML trigger is written for that table. The main function of DML triggers is to enforce business rules and extend SQL Server constraints, default values, etc. This is because constraints can only constrain data within the same table, while triggers can execute any SQL command.
DDL Triggers:These are triggers introduced in SQL Server 2005, primarily used to audit and regulate operations on tables, triggers, views, and other structures within the database. Examples include modifying tables, columns, adding tables, and adding columns. They execute when the database structure changes. We mainly use them to record database modifications and restrict programmers' modifications to the database, such as disallowing the deletion of certain specified tables.
Login Triggers:Login triggers fire stored procedures in response to the LOGIN event. This event is raised when a user session is established with the SQL Server instance. Login triggers fire after the login authentication phase is complete but before the user session is actually established. Therefore, all messages from within the trigger that will typically reach the user (such as error messages and messages from PRINT statements) are sent to the SQL Server error log. Login triggers will not fire if authentication fails.
Batteries:In the late 1980s, the United States applied toroidal inductive ballasts to compact energy-saving fluorescent lamps. Midwest Toriod began mass production in 1988. my country subsequently began production in the early 1990s. A ballast resistor is a device in a fluorescent lamp that limits current and generates instantaneous high voltage. It is made by winding enameled wire around a silicon steel core. This coil with an iron core generates a high voltage through self-induction when power is switched on and off instantaneously, which is applied to the electrodes (filaments) at both ends of the fluorescent tube. This action occurs alternately. When the starter (bulb jumper) is closed, the filament of the tube conducts and heats up through the ballast's current-limiting mechanism. When the starter is open, the ballast generates a high voltage through self-induction, which is applied to the filaments at both ends of the tube. The filaments emit electrons that bombard the phosphor on the tube wall, causing it to emit light. This repeated switching of the starter on and off several times completes the cycle, thus turning on the tube. When the tube is emitting light normally, its internal resistance decreases, and the starter remains open, allowing a stable current to flow through the tube and ballast, enabling the tube to emit light normally. Because a ballast constantly carries current during fluorescent lamp operation, it is prone to vibration and heat generation. Therefore, fluorescent lamps with ballasts, especially those with poor-quality ballasts, can produce significant noise and are prone to burning out over time. Ballasts are divided into electronic ballasts and inductive ballasts.
Inductive Ballast:When a 220V 50Hz AC power supply is applied to the circuit after the switch is closed, current flows through the ballast. The lamp filament starter heats the filament (initially the starter is open; due to the applied AC voltage greater than 190V, an arc discharge occurs in the bulb within the starter, causing the bimetallic strip to heat and deform, bringing the two electrodes together to form a circuit for heating the filament). When the two electrodes of the starter come together, the bimetallic strip cools due to the absence of arc discharge, and the electrodes separate. Because the inductive ballast is inductive, when the circuit is suddenly interrupted, a pulse voltage of 600V-1500V lasting approximately 1ms is generated across the lamp. The exact voltage value depends on the type of lamp. During the discharge, the voltage across the lamp drops immediately. At this time, the ballast limits the lamp current and creates a 55° to 65° phase difference between the power supply voltage and the lamp's operating current, thus maintaining the lamp's secondary starting voltage and enabling more stable lamp operation.
Inductive ballasts, due to their simple structure, were the first type of ballast used with fluorescent lamps and still held a relatively large market share. However, due to their low power factor, poor low-voltage starting performance, heavy and energy-intensive operation, flickering, and other drawbacks, their market share was gradually replaced by electronic ballasts. The energy loss of an inductive ballast is: 40W (lamp power) + 10W (heat loss from the inductive ballast itself) equals a total power consumption of 50W for the entire lighting system.
Electronic Ballasts:An electronic ballast is a converter that transforms mains frequency AC power into high-frequency AC power. Its basic working principle is as follows:The mains frequency power passes through a radio frequency interference (RFI) filter, full-wave rectification, and a passive (or active) power factor corrector (PPFC or APFC) to become DC power. A high-frequency AC power supply of 20K-100KHZ is output through a DC/AC converter and applied to the LC series resonant circuit connected to the lamp to heat the filament. This causes the lamp to transition from a "discharge" to a "conduction" state before illuminating. At this point, the high-frequency inductor limits the increase in current, ensuring the lamp receives the necessary voltage and current for normal operation. To improve reliability, various protection circuits are often added, such as fault protection, surge voltage and current protection, and temperature protection.
The difference between a trigger and a ballast: The ballast itself is not designed to generate high voltage. Due to lifespan considerations, the coil of the electronic trigger is placed in the ballast. This extends the trigger's lifespan and saves costs. Furthermore, simply generating high voltage is insufficient; continuous high voltage would damage the bulb. Could a bulb withstand several thousand volts of voltage continuously? Therefore, the electronic trigger stops generating high voltage after the bulb lights up and resumes generating high voltage after the bulb goes out.
Inductive ballasts require a trigger and a compensation capacitor to function. The trigger's function is to activate the high-pressure gas in the lamp momentarily upon power-on, lighting the lamp, and then disengage. Electronic ballasts do not require a trigger.
