一个健壮的实现并不容易。对于本题提出的有限问题,解决方法比较简单——强调“相对”。我假设如下:
- 查询只有一个级别——没有 UNION、子查询、WITH 表达式或其他为别名引入新范围的东西(这会很快变得复杂)。
- 查询中的所有标识符都是完全限定的,因此毫无疑问它指的是什么对象。
解决策略是这样的:我们首先访问TSqlFragment
以列出所有表别名,然后再次访问以获取所有等值连接,并在此过程中扩展别名。使用该列表,我们确定不引用同一数据库的等值连接列表。在代码中:
var sql = @"
UPDATE t3
SET description = 'abc'
FROM database1.dbo.table1 t1
INNER JOIN database2.dbo.table2 t2
ON (t1.id = t2.t1_id)
LEFT OUTER JOIN database3.dbo.table3 t3
ON (t3.id = t2.t3_id)
INNER JOIN database2.dbo.table4 t4
ON (t4.id = t2.t4_id)
";
var parser = new TSql120Parser(initialQuotedIdentifiers: false);
IList<ParseError> errors;
TSqlScript script;
using (var reader = new StringReader(sql)) {
script = (TSqlScript) parser.Parse(reader, out errors);
}
// First resolve aliases.
var aliasResolutionVisitor = new AliasResolutionVisitor();
script.Accept(aliasResolutionVisitor);
// Then find all equijoins, expanding aliases along the way.
var findEqualityJoinVisitor = new FindEqualityJoinVisitor(
aliasResolutionVisitor.Aliases
);
script.Accept(findEqualityJoinVisitor);
// Now list all aliases where the left database is not the same
// as the right database.
foreach (
var equiJoin in
findEqualityJoinVisitor.EqualityJoins.Where(
j => !j.JoinsSameDatabase()
)
) {
Console.WriteLine(equiJoin.ToString());
}
输出:
database3.dbo.table3.id = database2.dbo.table2.t3_id
database1.dbo.table1.id = database2.dbo.table2.t1_id
AliasResolutionVisitor
是一件简单的事情:
public class AliasResolutionVisitor : TSqlFragmentVisitor {
readonly Dictionary<string, string> aliases = new Dictionary<string, string>();
public Dictionary<string, string> Aliases { get { return aliases; } }
public override void Visit(NamedTableReference namedTableReference ) {
Identifier alias = namedTableReference.Alias;
string baseObjectName = namedTableReference.SchemaObject.AsObjectName();
if (alias != null) {
aliases.Add(alias.Value, baseObjectName);
}
}
}
我们只需遍历查询中的所有命名表引用,如果它们有别名,则将其添加到字典中。请注意,如果引入子查询,这将非常失败,因为此访问者没有范围的概念(实际上,为访问者添加范围要困难得多,因为TSqlFragment
没有提供注释解析树甚至从节点遍历它的方法)。
EqualityJoinVisitor
更有趣的是:
public class FindEqualityJoinVisitor : TSqlFragmentVisitor {
readonly Dictionary<string, string> aliases;
public FindEqualityJoinVisitor(Dictionary<string, string> aliases) {
this.aliases = aliases;
}
readonly List<EqualityJoin> equalityJoins = new List<EqualityJoin>();
public List<EqualityJoin> EqualityJoins { get { return equalityJoins; } }
public override void Visit(QualifiedJoin qualifiedJoin) {
var findEqualityComparisonVisitor = new FindEqualityComparisonVisitor();
qualifiedJoin.SearchCondition.Accept(findEqualityComparisonVisitor);
foreach (
var equalityComparison in findEqualityComparisonVisitor.Comparisons
) {
var firstColumnReferenceExpression =
equalityComparison.FirstExpression as ColumnReferenceExpression
;
var secondColumnReferenceExpression =
equalityComparison.SecondExpression as ColumnReferenceExpression
;
if (
firstColumnReferenceExpression != null &&
secondColumnReferenceExpression != null
) {
string firstColumnResolved = resolveMultipartIdentifier(
firstColumnReferenceExpression.MultiPartIdentifier
);
string secondColumnResolved = resolveMultipartIdentifier(
secondColumnReferenceExpression.MultiPartIdentifier
);
equalityJoins.Add(
new EqualityJoin(firstColumnResolved, secondColumnResolved)
);
}
}
}
private string resolveMultipartIdentifier(MultiPartIdentifier identifier) {
if (
identifier.Identifiers.Count == 2 &&
aliases.ContainsKey(identifier.Identifiers[0].Value)
) {
return
aliases[identifier.Identifiers[0].Value] + "." +
identifier.Identifiers[1].Value;
} else {
return identifier.AsObjectName();
}
}
}
这会寻找QualifiedJoin
实例,如果我们找到它们,我们会依次检查搜索条件以查找所有出现的相等比较。请注意,这确实适用于嵌套搜索条件:在 中Bar JOIN Foo ON Bar.Quux = Foo.Quux AND Bar.Baz = Foo.Baz
,我们将找到两个表达式。
我们如何找到它们?使用另一个小访客:
public class FindEqualityComparisonVisitor : TSqlFragmentVisitor {
List<BooleanComparisonExpression> comparisons =
new List<BooleanComparisonExpression>()
;
public List<BooleanComparisonExpression> Comparisons {
get { return comparisons; }
}
public override void Visit(BooleanComparisonExpression e) {
if (e.IsEqualityComparison()) comparisons.Add(e);
}
}
这里没有什么复杂的。将此代码折叠到其他访问者中并不难,但我认为这更清楚。
就是这样,除了一些我将不加评论地呈现的帮助代码:
public class EqualityJoin {
readonly SchemaObjectName left;
public SchemaObjectName Left { get { return left; } }
readonly SchemaObjectName right;
public SchemaObjectName Right { get { return right; } }
public EqualityJoin(
string qualifiedObjectNameLeft, string qualifiedObjectNameRight
) {
var parser = new TSql120Parser(initialQuotedIdentifiers: false);
IList<ParseError> errors;
using (var reader = new StringReader(qualifiedObjectNameLeft)) {
left = parser.ParseSchemaObjectName(reader, out errors);
}
using (var reader = new StringReader(qualifiedObjectNameRight)) {
right = parser.ParseSchemaObjectName(reader, out errors);
}
}
public bool JoinsSameDatabase() {
return left.Identifiers[0].Value == right.Identifiers[0].Value;
}
public override string ToString() {
return String.Format("{0} = {1}", left.AsObjectName(), right.AsObjectName());
}
}
public static class MultiPartIdentifierExtensions {
public static string AsObjectName(this MultiPartIdentifier multiPartIdentifier) {
return string.Join(".", multiPartIdentifier.Identifiers.Select(i => i.Value));
}
}
public static class ExpressionExtensions {
public static bool IsEqualityComparison(this BooleanExpression expression) {
return
expression is BooleanComparisonExpression &&
((BooleanComparisonExpression) expression).ComparisonType == BooleanComparisonType.Equals
;
}
}
正如我之前提到的,这段代码非常脆弱。它假定查询具有特定的形式,如果不这样做,它可能会失败(非常糟糕,会给出误导性的结果)。一个主要的开放挑战是扩展它,以便它可以正确处理范围和非限定引用,以及 T-SQL 脚本可能具有的其他怪异特性,但我认为它仍然是一个有用的起点。