/// <summary>
/// For CASE statements which represent boolean values:
///
/// CASE XXX
/// WHEN AAA THEN true ===> (XXX==AAA) || (XXX==BBB)
/// WHEN BBB THEN true
/// ELSE false
/// etc.
/// END
///
/// Also,
///
/// CASE XXX
/// WHEN AAA THEN false ===> (XXX!=AAA) && (XXX!=BBB)
/// WHEN BBB THEN false
/// ELSE true
/// etc.
/// END
///
/// The reduce to a conjunction or disjunction of equality or inequality.
/// The possibility of NULL in XXX is taken into account.
/// </summary>
private SqlExpression TryToWriteAsSimpleBooleanExpression(Type caseType, SqlExpression discriminator, List <SqlWhen> newWhens, bool allValuesLiteral)
{
SqlExpression rewrite = null;
if (caseType == typeof(bool) && allValuesLiteral)
{
bool?holdsNull = SqlExpressionNullability.CanBeNull(discriminator);
// The discriminator can't hold a NULL.
// In this case, we don't need the special fallback that CASE-ELSE gives.
// We can just construct a boolean operation.
bool?whenValue = null;
for (int i = 0; i < newWhens.Count; ++i)
{
SqlValue lit = (SqlValue)newWhens[i].Value; // Must be SqlValue because of allValuesLiteral.
bool value = (bool)lit.Value; // Must be bool because of caseType==typeof(bool).
if (newWhens[i].Match != null)
{ // Skip the ELSE
if (value)
{
rewrite = _sql.OrAccumulate(rewrite, _sql.Binary(SqlNodeType.EQ, discriminator, newWhens[i].Match));
}
else
{
rewrite = _sql.AndAccumulate(rewrite, _sql.Binary(SqlNodeType.NE, discriminator, newWhens[i].Match));
}
}
else
{
whenValue = value;
}
}
// If it could possibly hold null values.
if (holdsNull != false && whenValue != null)
{
if (whenValue == true)
{
rewrite = _sql.OrAccumulate(rewrite, _sql.Unary(SqlNodeType.IsNull, discriminator, discriminator.SourceExpression));
}
else
{
rewrite = _sql.AndAccumulate(rewrite, _sql.Unary(SqlNodeType.IsNotNull, discriminator, discriminator.SourceExpression));
}
}
}
return(rewrite);
}
