Conditional judgment statements in sql

There are two common conditional judgment statements in SQL: IF-ELSE statement and CASE statement. The IF-ELSE statement executes different statements depending on whether the condition is true or false, while the CASE statement executes the corresponding statement depending on whether the expression value matches different conditions.

Conditional judgment statement in SQL

Conditional judgment statement is evaluated in SQL Statements that condition and take different actions based on the results. They are widely used in SQL queries and operations to filter data from the database, modify data, or control processes.

Type

There are two commonly used conditional judgment statements in SQL:

• IF-ELSE statement: Evaluates a condition and executes one statement if the condition is true, otherwise executes another statement.
• CASE Statement: Evaluate a condition and execute the corresponding statement based on different condition values.

Syntax

IF-ELSE statement

<code class="sql">IF <condition> THEN
  <statement_if_true>
ELSE
  <statement_if_false>
END IF;</code>

CASE statement

<code class="sql">CASE <expression>
  WHEN <case_value1> THEN <statement1>
  WHEN <case_value2> THEN <statement2>
  ...
  ELSE <default_statement>
END CASE;</code>

Example

Use the IF-ELSE statement to check if the value is greater than 5

<code class="sql">SELECT *
FROM table_name
WHERE value IF value > 5 THEN 'Greater than 5' ELSE 'Less than or equal to 5' END IF;</code>

Use the CASE statement to check the score range

<code class="sql">CASE score
  WHEN 90 TO 100 THEN '优秀'
  WHEN 80 TO 89 THEN '良好'
  WHEN 70 TO 79 THEN '中等'
  ELSE '不及格'
END CASE;</code>

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