implement IfElseStatement for MySQL backend

src/backend/query_builder.rs

@@ -387,9 +387,24 @@ pub trait QueryBuilder:
             SimpleExpr::Constant(val) => {
                 self.prepare_constant(val, sql);
             }
+            SimpleExpr::IfElse(val) => {
+                self.prepare_if_else_statement(val, sql);
+            }
         }
     }
 
+    fn prepare_if_else_statement(&self, val: &Box<IfElseStatement>, sql: &mut dyn SqlWriter) {
+        write!(sql, "IF ").unwrap();
+        self.prepare_simple_expr(&val.when, sql);
+        write!(sql, " THEN\n").unwrap();
+        self.prepare_simple_expr(&val.then, sql);
+        if let Some(otherwise) = &val.otherwise {
+            write!(sql, "\nELSE\n").unwrap();
+            self.prepare_simple_expr(otherwise, sql);
+        };
+        write!(sql, "\nEND IF").unwrap();
+    }
+
     /// Translate [`CaseStatement`] into SQL statement.
     fn prepare_case_statement(&self, stmts: &CaseStatement, sql: &mut dyn SqlWriter) {
         write!(sql, "(CASE").unwrap();

src/expr.rs

@@ -4,7 +4,7 @@
 //!
 //! [`SimpleExpr`] is the expression common among select fields, where clauses and many other places.
 
-use crate::{func::*, query::*, types::*, value::*};
+use crate::{func::*, if_else::*, query::*, types::*, value::*};
 
 /// Helper to build a [`SimpleExpr`].
 #[derive(Debug, Clone)]
@@ -35,6 +35,7 @@ pub enum SimpleExpr {
     AsEnum(DynIden, Box<SimpleExpr>),
     Case(Box<CaseStatement>),
     Constant(Value),
+    IfElse(Box<IfElseStatement>),
 }
 
 /// "Operator" methods for building complex expressions.

src/if_else.rs

@@ -0,0 +1,33 @@
+use crate::{QueryBuilder, SimpleExpr};
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct IfElseStatement {
+    pub when: SimpleExpr,
+    pub then: SimpleExpr,
+    pub otherwise: Option<SimpleExpr>,
+}
+
+impl IfElseStatement {
+    pub fn new(when: SimpleExpr, then: SimpleExpr, otherwise: Option<SimpleExpr>) -> Self {
+        Self {
+            when,
+            then,
+            otherwise,
+        }
+    }
+
+    pub fn to_string<T: QueryBuilder>(&self, query_builder: T) -> String {
+        let mut sql = String::with_capacity(256);
+        query_builder.prepare_if_else_statement(&Box::new(self.clone()), &mut sql);
+        sql
+    }
+}
+pub trait IfElseStatementBuilder {
+    /// Build corresponding SQL statement for certain database backend and return SQL string
+    fn build<T: QueryBuilder>(&self, query_builder: T) -> String;
+
+    /// Build corresponding SQL statement for certain database backend and return SQL string
+    fn to_string<T: QueryBuilder>(&self, query_builder: T) -> String {
+        self.build(query_builder)
+    }
+}

src/lib.rs

@@ -818,6 +818,7 @@ pub mod expr;
 pub mod extension;
 pub mod foreign_key;
 pub mod func;
+pub mod if_else;
 pub mod index;
 pub mod prepare;
 pub mod query;
@@ -835,6 +836,7 @@ pub use backend::*;
 pub use expr::*;
 pub use foreign_key::*;
 pub use func::*;
+pub use if_else::*;
 pub use index::*;
 pub use prepare::*;
 pub use query::*;

tests/mysql/if_else.rs

@@ -0,0 +1,45 @@
+use super::*;
+use pretty_assertions::assert_eq;
+
+#[rustfmt::skip]
+#[test]
+fn if_without_else() {
+    let query = Query::select().column(Asterisk).from(Glyph::Table).take();
+    let then = SimpleExpr::SubQuery(None, Box::new(query.into_sub_query_statement()));
+    let if_statement = IfElseStatement::new(
+        Expr::col(Glyph::Id).eq(1),
+        then,
+        None
+    );
+    assert_eq!(
+        if_statement.to_string(MysqlQueryBuilder),
+        [
+            "IF `id` = 1 THEN",
+            "(SELECT * FROM `glyph`)",
+            "END IF"
+        ].join("\n")
+    )
+}
+
+#[rustfmt::skip]
+#[test]
+fn if_with_else() {
+    let query = Query::select().column(Asterisk).from(Glyph::Table).take();
+    let then = SimpleExpr::SubQuery(None, Box::new(query.into_sub_query_statement()));
+    let if_statement = IfElseStatement::new(
+        Expr::col(Glyph::Id).eq(1),
+        then,
+        Some(Expr::val("23").into()),
+    );
+    assert_eq!(
+        if_statement.to_string(MysqlQueryBuilder),
+        [
+            "IF `id` = 1 THEN",
+            "(SELECT * FROM `glyph`)",
+            "ELSE",
+            "'23'",
+            "END IF"
+        ]
+        .join("\n")
+    )
+}

tests/mysql/mod.rs

@@ -1,6 +1,7 @@
 use sea_query::{extension::mysql::*, tests_cfg::*, *};
 
 mod foreign_key;
+mod if_else;
 mod index;
 mod query;
 mod table;