mybatis中sql標籤重要是為了避免在專案開發的過程中重複編寫大量相同的sql語句，例如下面的查詢語句：

    <select id="selectCountryAndCity" parameterType="map" resultMap="countryAndCity1">
        SELECT country.Code as country_code,
        country.Name AS country_name,
        country.Continent AS country_contient,
        country.Region as
 
 country_region,
        country.SurfaceArea as country_surfaceArea,
        country.IndepYear as country_indepYear,
        country.Population as country_population,
        country.LifeExpectancy as country_lifeExpectancy,
        country.GNP as country_gnp,
        country.GNPOld as country_gnpOld,
        country.LocalName as
 
 country_localName,
        country.GovernmentForm as country_governmentForm,
        country.HeadOfState as country_headOfState,
        country.Capital as country_capital,
        country.Code2 as country_code2,
        city.ID as city_id,
        city.Name as city_name,
        city.CountryCode as
 
 city_countryCode,
        city.District as city_district,
        city.Population as city_population
        From country LEFT JOIN city ON country.code=city.countrycode WHERE country.code=#{code} AND city.name=#{name}

    </select>
    <select id="selectAllCityByCountry" parameterType="map" resultMap="countryAndCity2">
        SELECT country.Code as country_code,
        country.Name AS country_name,
        country.Continent AS country_contient,
        country.Region as country_region,
        country.SurfaceArea as country_surfaceArea,
        country.IndepYear as country_indepYear,
        country.Population as country_population,
        country.LifeExpectancy as country_lifeExpectancy,
        country.GNP as country_gnp,
        country.GNPOld as country_gnpOld,
        country.LocalName as country_localName,
        country.GovernmentForm as country_governmentForm,
        country.HeadOfState as country_headOfState,
        country.Capital as country_capital,
        country.Code2 as country_code2,
        city.ID as city_id,
        city.Name as city_name,
        city.CountryCode as city_countryCode,
        city.District as city_district,
        city.Population as city_population
        From country LEFT JOIN city ON country.code=city.countrycode WHERE country.code=#{code}

    </select>
    <select id="selectResultByIndepYear" parameterType="map" resultMap="countryAndCity3">
        SELECT country.Code as country_code,
        country.Name AS country_name,
        country.Continent AS country_contient,
        country.Region as country_region,
        country.SurfaceArea as country_surfaceArea,
        country.IndepYear as country_indepYear,
        country.Population as country_population,
        country.LifeExpectancy as country_lifeExpectancy,
        country.GNP as country_gnp,
        country.GNPOld as country_gnpOld,
        country.LocalName as country_localName,
        country.GovernmentForm as country_governmentForm,
        country.HeadOfState as country_headOfState,
        country.Capital as country_capital,
        country.Code2 as country_code2,
        city.ID as city_id,
        city.Name as city_name,
        city.CountryCode as city_countryCode,
        city.District as city_district,
        city.Population as city_population
        From country LEFT JOIN city ON country.code=city.countrycode WHERE country.code=#{code}

    </select>

除了最後的查詢條件或者返回的resultMap出現變化之外，需要查詢的欄位是完全相同的，通過使用sql標籤，可以將所有需要查詢的欄位封裝在一個sql語句中，語句如下：

    <sql id="all_columns">
        country.Code as country_code,
        country.Name AS country_name,
        country.Continent AS country_contient,
        country.Region as country_region,
        country.SurfaceArea as country_surfaceArea,
        country.IndepYear as country_indepYear,
        country.Population as country_population,
        country.LifeExpectancy as country_lifeExpectancy,
        country.GNP as country_gnp,
        country.GNPOld as country_gnpOld,
        country.LocalName as country_localName,
        country.GovernmentForm as country_governmentForm,
        country.HeadOfState as country_headOfState,
        country.Capital as country_capital,
        country.Code2 as country_code2,
        city.ID as city_id,
        city.Name as city_name,
        city.CountryCode as city_countryCode,
        city.District as city_district,
        city.Population as city_population
    </sql>

那麼通過使用sql將所有需要查詢的欄位封裝起來，在編寫select語句的時候就不需要每次都重複編寫需要查詢的欄位了，只需要在查詢語句中包含定義的sql語句即可，改造後的查詢語句如下：

    <select id="selectCountryAndCity" parameterType="map" resultMap="countryAndCity1">
        SELECT <include refid="all_columns" />
        From country LEFT JOIN city ON country.code=city.countrycode WHERE country.code=#{code} AND city.name=#{name}
    </select>
    <select id="selectAllCityByCountry" parameterType="map" resultMap="countryAndCity2">
        SELECT <include refid="all_columns" />
        From country LEFT JOIN city ON country.code=city.countrycode WHERE country.code=#{code}
    </select>
    <select id="selectResultByIndepYear" parameterType="map" resultMap="countryAndCity3">
        SELECT <include refid="all_columns" />
        From country LEFT JOIN city ON country.code=city.countrycode WHERE country.code=#{code}
    </select>

通過使用sql標籤減少重複的sql語句，不僅可以減少我們的程式碼量，還更加方便我們檢視！

測試中所使用的資料庫可以通過點選world.sql下載