Add support for check constraints.

src/main/kotlin/org/jetbrains/exposed/sql/Constraints.kt

@@ -76,17 +76,17 @@ data class CheckConstraint(val tableName: String, val checkName: String, val che
     }
 
     override fun createStatement(): List<String> {
-        if (currentDialect is MysqlDialect) throw UnsupportedOperationException("Check constraints are not currently supported by MySQL")
+        if (currentDialect is MysqlDialect) exposedLogger.warn("CHECK constraints are not currently supported by MySQL")
         return listOf("ALTER TABLE $tableName ADD$checkPart")
     }
 
     override fun dropStatement(): List<String> {
-        if (currentDialect is MysqlDialect) throw UnsupportedOperationException("Check constraints are not currently supported by MySQL")
+        if (currentDialect is MysqlDialect) throw UnsupportedOperationException("CHECK constraints are not currently supported by MySQL")
         return listOf("ALTER TABLE $tableName DROP CONSTRAINT $checkName")
     }
 
     override fun modifyStatement(): List<String> {
-        if (currentDialect is MysqlDialect) throw UnsupportedOperationException("Check constraints are not currently supported by MySQL")
+        if (currentDialect is MysqlDialect) throw UnsupportedOperationException("CHECK constraints are not currently supported by MySQL")
         return dropStatement() + createStatement()
     }
 }

src/main/kotlin/org/jetbrains/exposed/sql/Table.kt

@@ -404,21 +404,25 @@ open class Table(name: String = ""): ColumnSet(), DdlAware {
     }
 
     /**
-     * Creates a check constraint in this column
-     * @param name The name to identify the constraint, optional.
-     * @param op The expression that the value of this column must satisfy
+     * Creates a check constraint in this column.
+     * @param name The name to identify the constraint, optional. Must be **unique** (case-insensitive) to this table, otherwise, the constraint will
+     * not be created. All names are [trimmed][String.trim], blank names are ignored and the database engine decides the default name.
+     * @param op The expression against which the newly inserted values will be compared.
      */
     fun <T> Column<T>.check(name: String = "", op: SqlExpressionBuilder.(Column<T>) -> Op<Boolean>) = apply {
-        table.checkConstraints.add(name to SqlExpressionBuilder.op(this))
+        table.checkConstraints.takeIf { name.isEmpty() || it.none { it.first.equals(name, true) } }?.add(name to SqlExpressionBuilder.op(this))
+                ?: exposedLogger.warn("A CHECK constraint with name '$name' was ignored because there is already one with that name")
     }
 
     /**
-     * Creates a check constraint in this table
-     * @param name The name to identify the constraint, optional.
-     * @param op The expression that the values in certain columns must satisfy
+     * Creates a check constraint in this table.
+     * @param name The name to identify the constraint, optional. Must be **unique** (case-insensitive) to this table, otherwise, the constraint will
+     * not be created. All names are [trimmed][String.trim], blank names are ignored and the database engine decides the default name.
+     * @param op The expression against which the newly inserted values will be compared.
      */
     fun check(name: String = "", op: SqlExpressionBuilder.() -> Op<Boolean>) {
-        checkConstraints.add(name to SqlExpressionBuilder.op())
+        checkConstraints.takeIf { name.isEmpty() || it.none { it.first.equals(name, true) } }?.add(name to SqlExpressionBuilder.op())
+                ?: exposedLogger.warn("A CHECK constraint with name '$name' was ignored because there is already one with that name")
     }
 
     val ddl: List<String>

src/test/kotlin/org/jetbrains/exposed/sql/tests/shared/DDLTests.kt

@@ -219,7 +219,7 @@ class DDLTests : DatabaseTestsBase() {
             else -> "NULL"
         }
 
-        
+
         withTables(TestTable) {
             val dtType = currentDialect.dataTypeProvider.dateTimeType()
             assertEquals("CREATE TABLE " + if (currentDialect.supportsIfNotExists) { "IF NOT EXISTS " } else { "" } +
@@ -384,11 +384,11 @@ class DDLTests : DatabaseTestsBase() {
         }
     }
 
-    
+
     private abstract class EntityTable(name: String = "") : IdTable<String>(name) {
         override val id: Column<EntityID<String>> = varchar("id", 64).clientDefault { UUID.randomUUID().toString() }.primaryKey().entityId()
     }
-    
+
     @Test fun complexTest01() {
         val User = object : EntityTable() {
             val name = varchar("name", 255)
@@ -482,10 +482,17 @@ class DDLTests : DatabaseTestsBase() {
 
             assertEquals(1, checkTable.selectAll().count())
 
-            assertFailAndRollback("Check constraint") {
+            assertFailAndRollback("Check constraint 1") {
                 checkTable.insert {
                     it[positive] = -472
-                    it[negative] = 354
+                    it[negative] = -354
+                }
+            }
+
+            assertFailAndRollback("Check constraint 2") {
+                checkTable.insert {
+                    it[positive] = 538
+                    it[negative] = 915
                 }
             }
         }
@@ -509,10 +516,17 @@ class DDLTests : DatabaseTestsBase() {
 
             assertEquals(1, checkTable.selectAll().count())
 
-            assertFailAndRollback("Check constraint") {
+            assertFailAndRollback("Check constraint 1") {
+                checkTable.insert {
+                    it[positive] = -47
+                    it[negative] = -35
+                }
+            }
+
+            assertFailAndRollback("Check constraint 2") {
                 checkTable.insert {
-                    it[positive] = -14
-                    it[negative] = 42
+                    it[positive] = 53
+                    it[negative] = 91
                 }
             }
         }