Lambda怎么实现动态SQL语句

Lambda如何实现动态SQL语句?
用Lambda如何实现如下SQL语句构建,if(a!=""){sql="a=123"} if(b!="")(sql+="b=123")

[解决办法]

C# code

var query = db.TableName;if (a != "")    query = query.Where(x => x.a == a);if (b != "")    query = query.Where(x => x.b == b);//query = db.TableName.Where(x => x.a == a && x.b == b);